From e033c450c0b9c069673ead3fcd2f5b424c3c863d Mon Sep 17 00:00:00 2001
From: Richi Jeffery <rici1717@gmail.com>
Date: Mon, 30 Jul 2018 14:22:01 -0700
Subject: [PATCH 01/46] fix errors that could trip people up const with arrow
 function missing = obj1.display() is not a function since display is not
 defined change to obj1.myMethod() fix some inconsistent indentation

---
 README.md | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/README.md b/README.md
index f248332..5e09ec9 100644
--- a/README.md
+++ b/README.md
@@ -21,7 +21,7 @@ Before starting further discussion let's understand the difference between decla
 
 ```javascript
 var x; // declaring x
-console.log(x); //output: undefined
+console.log(x); // output: undefined
 ```
 
 `var x = 1` is both declaration and definition (also we can say we are doing initialisation), Here declaration and assignment of value happen inline for variable x, In JavaScript every variable declaration and function declaration brings to the top of its current scope in which it's declared then assignment happen in order this term is called `hoisting`.
@@ -30,7 +30,7 @@ A variable can be declared but not defined. When we try to access it, It will re
 
 ```javascript
 var x; // Declaration
-if(typeof x === 'undefined') // Will return true
+typeof x === 'undefined'; // Will return true
 ```
 
 A variable can be neither declared nor defined. When we try to reference such variable then the result will be `not defined`.
@@ -94,9 +94,9 @@ Employee.prototype.displayIncreasedSalary = function() {
 };
   
 // Creating three Employee objects
-var emp1 = new Employee("John","Pluto",3000);
-var emp2 = new Employee("Merry","Pluto",2000);
-var emp3 = new Employee("Ren","Pluto",2500);
+var emp1 = new Employee("John", "Pluto", 3000);
+var emp2 = new Employee("Merry", "Pluto", 2000);
+var emp3 = new Employee("Ren", "Pluto", 2500);
 ```
 
 Here each instance variable `emp1`, `emp2`, `emp3` has own copy of increaseSalary private method. However the `displayIncreasedSalary` will only be added once to an object `Employee.prototype`.
@@ -831,7 +831,7 @@ A function can take a form of immediately invoked function expression (IIFE):
 Finally there are also arrow functions: 
 
 ```javascript
-const myFunc (arg) => {
+const myFunc = arg => {
     console.log("hello", arg)
 } 
 ```
@@ -850,7 +850,7 @@ var obj1 = {
 };
 
 // Call the method
-obj1.display();
+obj1.myMethod();
 ```
 
 Here `obj1` is an object and `myMethod` is a method which is associated with `obj1`.
@@ -861,7 +861,7 @@ In ES6 we have classes. There the methods will look like this:
 ```javascript
 class MyAwesomeClass {
   myMethod() {
-        console.log("hi there");
+    console.log("hi there");
   }
 }
 
@@ -873,10 +873,10 @@ Understand: the method is not some kind of special type of a function, and it's
 
 ```javascript
 var obj1 = {
-    prop1: "buddy"
+  prop1: "buddy"
 }; 
 var myFunc = function () {
-    console.log("Hi there", this);
+  console.log("Hi there", this);
 };
 // let's call myFunc as a function: 
 myFunc(); // will output "Hi there undefined" or "Hi there Window"

From a44c02e482a05e43fc0232e59b7cd84e539f0b62 Mon Sep 17 00:00:00 2001
From: rajesh <getmetorajesh@gmail.com>
Date: Fri, 3 Aug 2018 04:27:04 +1000
Subject: [PATCH 02/46] add missing new keyword in q23 answer

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index f248332..50c104e 100644
--- a/README.md
+++ b/README.md
@@ -786,7 +786,7 @@ function User(name) {
 }
 
 var person;
-var foo = User("xyz");
+var foo = new User("xyz");
 foo["location"] = "USA";
 // the console will show you that the result of this is "USA"
 

From c600226610bedad43d2de1b3739787bdfa28d298 Mon Sep 17 00:00:00 2001
From: Divyanshu Rawat <divyanshu.r46956@gmail.com>
Date: Tue, 14 Aug 2018 01:10:30 +0530
Subject: [PATCH 03/46] Question 21 Spelling Correction

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index ca588c5..ef3ab25 100644
--- a/README.md
+++ b/README.md
@@ -653,7 +653,7 @@ name instanceof String; // Output : true
 
 Ref Link: [http://stackoverflow.com/questions/2449254/what-is-the-instanceof-operator-in-javascript](http://stackoverflow.com/questions/2449254/what-is-the-instanceof-operator-in-javascript)
 
-## Question 21. Calculate the lengh of the associative array
+## Question 21. Calculate the length of the associative array
 
 ```javascript
 var counterArray = {

From e22d78c1682b8082c9c68087eb49a32bfb362c9b Mon Sep 17 00:00:00 2001
From: Samar Panda <er.samarpanda@gmail.com>
Date: Tue, 14 Aug 2018 05:29:22 +0530
Subject: [PATCH 04/46] Typo there are 7 primitive data types instead of 6

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index ca588c5..91662a7 100644
--- a/README.md
+++ b/README.md
@@ -632,7 +632,7 @@ var salary = "1000$";
 
 `typeof` is an operator that returns a string with the type of whatever you pass.
 
-The `typeof` operator checks if a value belongs to one of the six basic types: `number`, `string`, `boolean`, `object`, `function` or `undefined`.
+The `typeof` operator checks if a value belongs to one of the seven basic types: `number`, `string`, `boolean`, `object`, `function`, `undefined` or `Symbol`.
 
 `typeof(null)` will return `object`.
 

From 3e1c2829db85105fe1390bec9be3d8c4d0e6bef8 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Tue, 14 Aug 2018 15:36:49 +0200
Subject: [PATCH 05/46] q3 removed unnessesary code

---
 README.md | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/README.md b/README.md
index e4b8334..f187ac2 100644
--- a/README.md
+++ b/README.md
@@ -80,23 +80,13 @@ var Employee = function (name, company, salary) {
   this.company = company || "";
   this.salary = salary || 5000;
 
-  // Private method
+  // We can create a private method like this
   var increaseSalary = function () {
     this.salary = this.salary + 1000;
   };
 
 };
 
-// adding the method to Employee's prototype
-Employee.prototype.displayIncreasedSalary = function() {
-  increaseSalary();
-  console.log(this.salary);
-};
-  
-// Creating three Employee objects
-var emp1 = new Employee("John", "Pluto", 3000);
-var emp2 = new Employee("Merry", "Pluto", 2000);
-var emp3 = new Employee("Ren", "Pluto", 2500);
 ```
 
 Here each instance variable `emp1`, `emp2`, `emp3` has own copy of increaseSalary private method. However the `displayIncreasedSalary` will only be added once to an object `Employee.prototype`.

From c9929e46f8e5f28c9821eef2c95c68976f47866e Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Tue, 14 Aug 2018 15:42:36 +0200
Subject: [PATCH 06/46] q17 removed bad practice

---
 README.md | 32 --------------------------------
 1 file changed, 32 deletions(-)

diff --git a/README.md b/README.md
index f187ac2..456751b 100644
--- a/README.md
+++ b/README.md
@@ -516,38 +516,6 @@ function bar() {
 </script>
 ```
 
-Another advantage of declaring the function using function expressions is that you can declare function based on certain condition, for example:
-
-```javascript
-<script>
-if(testCondition) { // If testCondition is true then
-  function foo() {
-    console.log("inside Foo with testCondition True value");
-  };
- } else {
-  function foo() {
-    console.log("inside Foo with testCondition false value");
-  };
- }
- </script>
-```
-
-But If you try to run similar code in the following format, it would result in an error
-
-```javascript
-<script>
-if (testCondition) {
-   var foo = function() {
-      console.log("inside Foo with testCondition True value");
-   };
- } else {
-    var foo = function() {
-      console.log("inside Foo with testCondition false value");
-   };
- }
- </script>
-```
-
 ## Question 18. what is function hoisting in JavaScript?
 **Function Expression**
 

From 10afe78aefe9b11b9459ada083a4d2c2541d803a Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Tue, 14 Aug 2018 15:50:16 +0200
Subject: [PATCH 07/46] q2 replaced thanks to @fssq1993 and @yairEO

---
 README.md | 25 ++++++++-----------------
 1 file changed, 8 insertions(+), 17 deletions(-)

diff --git a/README.md b/README.md
index 456751b..b3739e4 100644
--- a/README.md
+++ b/README.md
@@ -43,30 +43,21 @@ console.log(y);  // Output: ReferenceError: y is not defined
 [http://stackoverflow.com/questions/20822022/javascript-variable-definition-declaration](http://stackoverflow.com/questions/20822022/javascript-variable-definition-declaration)
 
 
-## Question 2. What will be the output of the following code?
+## Question 2. For which value of `x` the results of the following statements are not the same?
+
 
 ```javascript
-var y = 1;
-if (function f() {}) {
-  y += typeof f;
-}
-console.log(y);
+//  if( x <= 100 ) {...}
+if( !(x > 100) ) {...}
 ```
 ### Answer 
 
-The code above would give the output `1undefined`. inside `if` statement we have `function f(){}`. Here's it's a function expression, not function statement, which is why f is not a variable. 
+`NaN <= 100` is `false` and `NaN > 100` is also `false`, so if the
+value of `x` is `NaN`, the statements are not the same.
 
-As a reminder, function expression is something like this: 
-```javascript
-var myFunc1 = new function() {}
-var myFunc2 = new function whatever() {} // whatever is not a variable here
-```
+The same holds true for any value of x that being converted to Number, returns NaN, e.g.: `undefined`, `[1,2,5]`, `{a:22}` , etc.
 
-A function statement is something like this: 
-```javascript
-function myFunc3() {}
-```
-You can read about [function expressions](https://developer.mozilla.org/en-US/docs/web/JavaScript/Reference/Operators/function) and [function statements](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function) on MDN.
+This is why you need to pay attention when you deal with numeric variables. `NaN` can’t be equal, less than or more than any other numeric value, so the only reliable way to check if the value is `NaN`, is to use `isNaN()` function.
 
 ## Question 3. What is the drawback of having private methods in a JavaScript object?
 

From 7fa59a4824bece9d26cb00ede848a0387ca3a7a0 Mon Sep 17 00:00:00 2001
From: AndroidFanatic <manish_raj@outlook.com>
Date: Mon, 20 Aug 2018 10:23:13 +0530
Subject: [PATCH 08/46] Update README.md

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index b3739e4..a6e2385 100644
--- a/README.md
+++ b/README.md
@@ -130,7 +130,7 @@ innerFuncVar = y
 globalVar = abc
 ```
 
-## Question 5. Write a mul function which will properly when invoked as below syntax.
+## Question 5. Write a mul function which will work properly when invoked with following syntax.
 
 ```javascript
 console.log(mul(2)(3)(4)); // output : 24

From e15b5d6306adbac9418896954a8a21953f4a0bb7 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sun, 2 Sep 2018 00:29:38 +0300
Subject: [PATCH 09/46] grammar fix

---
 README.md | 52 ++++++++++++++++++++++++++--------------------------
 1 file changed, 26 insertions(+), 26 deletions(-)

diff --git a/README.md b/README.md
index a6e2385..543cb42 100644
--- a/README.md
+++ b/README.md
@@ -2645,7 +2645,7 @@ console.log(obj.innerMessage);
 	
 Answer: 4) undefined true
 
-### 3. What would the output of following code ?
+### 3. What would be the output of following code ?
 
 ```javascript
 var obj = {
@@ -2665,7 +2665,7 @@ console.log(obj.innerMessage());
 	
 Answer: 1) Hello
 
-### 4. What would the output of following code ?
+### 4. What would be the output of following code ?
 
 ```javascript
 var obj = {
@@ -2686,7 +2686,7 @@ console.log(obj.innerMessage());
 	
 Answer: 3) undefined
 	
-### 5. What would the output of following code ?
+### 5. What would be the output of following code ?
 
 ```javascript
 var obj = {
@@ -2708,7 +2708,7 @@ console.log(obj.innerMessage());
 	
 Answer: 2) 'Hello'
 
-### 6. What would the output of following code ?
+### 6. What would be the output of following code ?
 
 ```javascript
 function myFunc(){
@@ -2726,7 +2726,7 @@ console.log(myFunc());
 	
 Answer: 3) undefined
 
-### 7. What would the output of following code ?
+### 7. What would be the output of following code ?
 
 ```javascript
 function myFunc(){
@@ -2744,7 +2744,7 @@ console.log(myFunc());
 	
 Answer: 2) 'Hi John'
 
-### 8. What would the output of following code ?
+### 8. What would be the output of following code ?
 
 ```javascript
 function myFunc() {
@@ -2761,7 +2761,7 @@ console.log(myFunc());
 	
 Answer: 2) 'Hi John'
 
-### 9. What would the output of following code ?
+### 9. What would be the output of following code ?
 
 ```javascript
 function myFunc(param1,param2) {
@@ -2779,7 +2779,7 @@ console.log(myFunc("a","b","c","d"));
 	
 Answer: a) 2 2 2 
 
-### 10. What would the output of following code ?
+### 10. What would be the output of following code ?
 
 ```javascript
 function myFunc() {
@@ -2799,7 +2799,7 @@ Answer: 2) 0 2 4
 
 ## Object Oriented
 
-### 1. What would the output of following code ?
+### 1. What would be the output of following code ?
 
 ```javascript
 function Person(name, age){
@@ -2829,7 +2829,7 @@ Answer: 1) John Person
 
 ## Scopes
 
-### 1. What would the output of following code ?
+### 1. What would be the output of following code ?
 
 ```javascript
 function passWordMngr() {
@@ -2852,7 +2852,7 @@ console.log(userInfo.userName);
 	
 Answer: 3) 12345678 undefined 
 
-### 2. What would the output of following code ?
+### 2. What would be the output of following code ?
 
 ```javascript
 var employeeId = 'aq123';
@@ -2869,7 +2869,7 @@ console.log(Employee.employeeId);
 	
 Answer: 4) undefined 
 
-### 3. What would the output of following code ?
+### 3. What would be the output of following code ?
 
 ```javascript
 var employeeId = 'aq123';
@@ -2891,7 +2891,7 @@ console.log(new Employee().employeeId);
 	
 Answer: 2) bq1uy 1BJKSJ bq1uy 
 
-### 4. What would the output of following code ?
+### 4. What would be the output of following code ?
 
 ```javascript
 var employeeId = 'aq123';
@@ -2914,7 +2914,7 @@ Answer: 1) foo123 aq123
 
 ## Call, Apply, Bind
 
-### 1. What would the output of following code ?
+### 1. What would be the output of following code ?
 
 ```javascript
 (function() {
@@ -2933,7 +2933,7 @@ Answer: 1) foo123 aq123
 	
 Answer: 4) [ 'W', 'o', 'r', 'l', 'd' ]  
 
-### 2. What would the output of following code ?
+### 2. What would be the output of following code ?
 
 ```javascript
 (function() {
@@ -2964,7 +2964,7 @@ Answer: 4) [ 'W', 'o', 'r', 'l', 'd' ]
 	
 Answer: 1) Total amount left in account: 5600 Total amount left in account: 5300 
 
-### 3. What would the output of following code ?
+### 3. What would be the output of following code ?
 
 ```javascript
 (function() {
@@ -2996,7 +2996,7 @@ Answer: 1) Total amount left in account: 5600 Total amount left in account: 5300
 	
 Answer: 1) 5600 5300 5100
 
-### 4. What would the output of following code ?
+### 4. What would be the output of following code ?
 
 ```javascript
 (function() {
@@ -3028,7 +3028,7 @@ Answer: 1) 5600 5300 5100
 	
 Answer: 2) 3600 3300 3100
 
-### 5. What would the output of following code ?
+### 5. What would be the output of following code ?
 
 ```javascript
 (function greetNewCustomer() {
@@ -3049,7 +3049,7 @@ Answer: 1) Hello John
 
 ## Callback Function
 
-### 1. What would the output of following code ?
+### 1. What would be the output of following code ?
 
 ```javascript
 function getDataFromServer(apiUrl){
@@ -3074,7 +3074,7 @@ getDataFromServer('www.google.com').then(function(name){
 	
 Answer: 1) John
 
-### 2. What would the output of following code ?
+### 2. What would be the output of following code ?
 
 ```javascript
 (function(){
@@ -3122,7 +3122,7 @@ Answer: 1) [ 2, 8, 15, 16, 23, 42 ]
 			
 ## Return Statement
 
-### 1. What would the output of following code ?
+### 1. What would be the output of following code ?
 
 ```javascript
 (function(){
@@ -3144,7 +3144,7 @@ Answer: 1) [ 2, 8, 15, 16, 23, 42 ]
 	
 Answer: 4) Uncaught TypeError: Cannot read property 'fullName' of undefined
 
-### 2. What would the output of following code ?
+### 2. What would be the output of following code ?
 
 ```javascript
 function getNumber(){
@@ -3162,7 +3162,7 @@ console.log(numb);
 	
 Answer: 1) 5
 
-### 3. What would the output of following code ?
+### 3. What would be the output of following code ?
 
 ```javascript
 function getNumber(){
@@ -3180,7 +3180,7 @@ console.log(numb);
 	
 Answer: 2) undefined
 
-### 4**. What would the output of following code ?
+### 4**. What would be the output of following code ?
 
 ```javascript
 function mul(x){
@@ -3202,7 +3202,7 @@ console.log(mul(2)(3)[1](4));
 	
 Answer: 1) 6, 10
 
-### 5**. What would the output of following code ?
+### 5**. What would be the output of following code ?
 
 ```javascript
 function mul(x) {
@@ -3226,7 +3226,7 @@ console.log(mul(2)(3).sum(4));
 	
 Answer: 1) 6, 10
 
-### 6. What would the output of following code ?
+### 6. What would be the output of following code ?
 
 ```javascript
 function mul(x) {

From be266526816ba6b4ccfee34668db4d75a3045ce8 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Tue, 25 Sep 2018 19:56:39 +0200
Subject: [PATCH 10/46] added more questions

---
 README.md | 202 +++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 161 insertions(+), 41 deletions(-)

diff --git a/README.md b/README.md
index 543cb42..5dd7f75 100644
--- a/README.md
+++ b/README.md
@@ -489,22 +489,18 @@ The main difference is function `foo` is defined at `run-time` and is called fun
 
 ```javascript
 // Run-Time function declaration
-<script>
   foo(); // Call foo function here, It will give an error
   var foo = function() {
     console.log("Hi I am inside Foo");
   };
-</script>
 ```
 
 ```javascript
 // Parse-Time function declaration
-<script>
 bar(); // Call bar function here, It will not give an Error
 function bar() {
   console.log("Hi I am inside Foo");
 };
-</script>
 ```
 
 ## Question 18. what is function hoisting in JavaScript?
@@ -1177,7 +1173,7 @@ Be sure that you can implement the promise, read [one of the articles on a topic
 
 ## Question 33. How to check whether a key exist in a JavaScript object or not.
 
->Let say we have `person` object with property **name** and **age**
+Let say we have `person` object with property **name** and **age**
 
 ```javascript
 var person = {
@@ -1263,7 +1259,7 @@ for (var i = 0; i < arr.length; i++) {
 
 ## Question 36. What is best way to detect an arrays object on JavaScript ?
 
->We always encounter in such situation where we need to know whether value is type of array or not.
+We always encounter in such situation where we need to know whether value is type of array or not.
 
 For Instance : Below code perform some operation based value type
 
@@ -1279,9 +1275,21 @@ function(value){
 
 Let's discuss some way to detect an array in JavaScript.
 
-**Method 1 :** 
+**Method 1:**
 
-Duck typing test for array type detection 
+Juriy Zaytsev (Also known as kangax) proposed an elegant solution to this.
+
+```javascript
+	function isArray(value){
+		return Object.prototype.toString.call(value) === '[object Array]';
+	}
+```
+This approach is most popular way to detecting a value of type array in JavaScript and recommended to use. This approach relies on the fact that, native toString() method on a given value produce a standard string in all browser. 
+
+
+**Method 2:** 
+
+Duck typing test for array type detection
 
 ```javascript
  // Duck typing arrays
@@ -1300,20 +1308,11 @@ As we can see above isArray method will return true if value object have `sort`
 ```
 Now when you check `isArray(bar)` then it will return true because bar object has sort method, But the fact is bar is not an array.
 
-So method 1 is not a best way to detect an array as you can see it's not handle the case when some object has sort method.
+So this method is not a best way to detect an array as you can see it's not handle the case when some object has sort method.
 
-**Method 2:**
+**Method 3:** 
 
-Juriy Zaytsev (Also known as kangax) proposed an elegant solution to this.
-
-```javascript
-	function isArray(value){
-		return Object.prototype.toString.call(value) === '[object Array]';
-	}
-```
-This approach is most popular way to detecting a value of type array in JavaScript and recommended to use. This approach relies on the fact that, native toString() method on a given value produce a standard string in all browser.
-
->ECMAScript 5 has introduced **Array.isArray()** method to detect an array type value. The sole purpose of this method is accurately detecting whether a value is an array or not.
+ECMAScript 5 has introduced **Array.isArray()** method to detect an array type value. The sole purpose of this method is accurately detecting whether a value is an array or not.
 
 In many JavaScript libraries you may see below code for detecting an value of type array.
 
@@ -1330,7 +1329,7 @@ function(value){
 
 ## Question 37. Best way to detect reference values of any type in JavaScript ?
 
-> In Javascript Object are called as reference type, Any value other then primitive is definitely a reference type. There are several built-in reference type such as **Object**, **Array**, **Function**, **Date**, **null** and **Error**.
+ In Javascript Object are called as reference type, Any value other then primitive is definitely a reference type. There are several built-in reference type such as **Object**, **Array**, **Function**, **Date**, **null** and **Error**.
 
 Detecting object using `typeof` operator
 
@@ -1372,7 +1371,7 @@ console.log(emp1 instanceof Object); // true
 
 ## Question 38. Describe Object-Based inheritance in JavaScript.
 
-> Object-based inheritance also called prototypal inheritance in which we one object inherit from another object without invoking a constructor function.
+ Object-based inheritance also called prototypal inheritance in which we one object inherit from another object without invoking a constructor function.
 
 The ECMAScript 5 **Object.create()** method is the easiest way for one object to inherit from another. 
 
@@ -1419,11 +1418,11 @@ employee.displayName(); // "Nishant"
 ```
 In above example, `emp1` is created with it's own value for name, so calling **displayName()** method display `"John"` instead of `"Nishant"`.
 
->Object created in this manner give you full control over newly created object. You are free to add, remove any properties and method you want.
+Object created in this manner give you full control over newly created object. You are free to add, remove any properties and method you want.
 
 ## Question 39. Describe Type-Based inheritance in JavaScript.
 
->Type-based inheritance works with constructor function instead of object, It means we need to call constructor function of the object from which you  want to inherit.
+Type-based inheritance works with constructor function instead of object, It means we need to call constructor function of the object from which you  want to inherit.
 
 Let say we have `Person` class which has name, age, salary properties and **incrementSalary()** method.
 
@@ -1484,7 +1483,7 @@ Type-based inheritance is best used with developer defined constructor function
 
 ## Question 40. How we can prevent modification of object in JavaScript ?.
 
-> ECMAScript 5 introduce several methods to prevent modification of object which lock down object to ensure that no one, accidentally or otherwise, change functionality of Object.
+ ECMAScript 5 introduce several methods to prevent modification of object which lock down object to ensure that no one, accidentally or otherwise, change functionality of Object.
 
 There are three levels of preventing modification: 
 
@@ -1510,7 +1509,7 @@ employee.age = 24; // fails silently unless it's inside the strict mode
 ```
 **2: Seal :**
 
->It is same as prevent extension, in addition to this also prevent existing properties and methods from being deleted.
+It is same as prevent extension, in addition to this also prevent existing properties and methods from being deleted.
 
 To seal an object, we use **Object.seal()** method. you can check whether an object is sealed or not using **Object.isSealed();**
 
@@ -1535,7 +1534,7 @@ when an object is sealed, its existing properties and methods can't be removed.
 
 **3: Freeze :**
 
->Same as seal, In addition to this prevent existing properties methods from being modified (All properties and methods are read only).
+Same as seal, In addition to this prevent existing properties methods from being modified (All properties and methods are read only).
 
 To freeze an object, use Object.freeze() method. We can also determine whether an object is frozen using Object.isFrozen();
 
@@ -1593,7 +1592,7 @@ delete employee.name;  // fails silently unless it's in strict mode
 
 
 ## Question 44. Write a log function which will add prefix `(your message)` to every message you log using console.log ? 
-> For example, If you log `console.log("Some message")` then output should be **(your message) Some message**
+ For example, If you log `console.log("Some message")` then output should be **(your message) Some message**
 
 Logging error message or some informative message is always required when you dealing with client side JavaScript using console.log method. Some time you want to add some prefix to identify message generated log from your application hence you would like to prefix your app name in every console.log. 
 
@@ -1627,7 +1626,7 @@ For example: We can create string using string literal and using String construc
  // using String constructor function 
  var objStr = new String("Hi I am string object");
 ```
-> We can use typeof operator to test string literal and instanceof operator to test String object.
+ We can use typeof operator to test string literal and instanceof operator to test String object.
 
 ```javascript
  function isString(str) {
@@ -1641,7 +1640,7 @@ For example: We can create string using string literal and using String construc
 ``` 
 ## Question 46 . What is typical use case for anonymous function in JavaScript ?
 
-> Anonymous functions basically used in following scenario.
+ Anonymous functions basically used in following scenario.
 
 1. No name is needed if function is only used in one place, then there is no need to add a name to function.
 
@@ -1691,9 +1690,9 @@ For example: We can create string using string literal and using String construc
 		alert("Hi I am anonymous callback function");
 	});
 	```
-The best way to take decision for using anonymous function is to asked.
+The best way to make a decision for using anonymous function is to ask the following question:
 
-> Will the function which I am going to define will use anywhere else.
+ Will the function which I am going to define, be used anywhere else?
 
 If your answer is yes then go and create named function rather anonymous function.
 
@@ -1704,7 +1703,7 @@ If your answer is yes then go and create named function rather anonymous functio
 
 ## Question 47 . How to set a default parameter value ?
 
-> If you are coming from python/c# you might be using default value for function parameter incase value(formal parameter) has not been passed. For Instance : 
+ If you are coming from python/c# you might be using default value for function parameter incase value(formal parameter) has not been passed. For Instance : 
 
 ```python
 // Define sentEmail function 
@@ -1762,7 +1761,7 @@ sentEmail({
 ```
 
 ## Question 48. Write code for merge two JavaScript Object dynamically.
-> Let say you have two object 
+Let say you have two objects 
 
 ```javascript
 var person = {
@@ -1787,7 +1786,7 @@ name , age , addressLine1 , addressLine2 , city */
 **Method 1: Using ES6, Object assign method**
 
 ```javascript
-const merge = (toObj, fromObj) => Object.assign(toObj, fromObj);
+const merge = (toObj, fromObj) = Object.assign(toObj, fromObj);
 ```
  
 **Method 2: Without using in-built function**
@@ -1810,7 +1809,7 @@ function merge(toObj, fromObj) {
 ```
 ## Question 49. What is non-enumerable property in JavaScript and how can create ?
 
->Object can have properties that don't show up when you iterate through object using for...in loop or using Object.keys() to get an array of property names. This properties is know as non-enumerable properties.
+Object can have properties that don't show up when you iterate through object using for...in loop or using Object.keys() to get an array of property names. This properties is know as non-enumerable properties.
 
 Let say we have following object
 
@@ -1857,7 +1856,7 @@ Changing non-enumerable property value will return error in `strict mode`. In no
 
 ## Question 50. What is Function binding ?
 
-> Function binding falls in advance JavaScript category and this is very popular technique to use in conjunction with event handler and callback function to preserve code execution context while passing function as a parameter.
+ Function binding falls in advance JavaScript category and this is very popular technique to use in conjunction with event handler and callback function to preserve code execution context while passing function as a parameter.
 
 Let's consider the following example:
 
@@ -1898,6 +1897,126 @@ btn.addEventListener('click', clickHandler.handleClick.bind(clickHandler));
 
 # Coding Questions
 
+## Passing values by reference vs by value
+For a JS developer, it's crucially important to understand which values are passed by reference,
+and which ones are passed by value. Remember that objects, including arrays are passed by reference
+while strings, booleans and numbers are passed by value. 
+
+### 1. What would be the output of following code?
+
+```javascript
+var strA = "hi there";
+var strB = strA;
+strB="bye there!";
+console.log (strA)
+```
+
+The output will `'hi there'` because we're dealing with strings here. Strings are 
+passed by value, that is, copied. 
+ 
+### 2. What would be the output of following code?
+```javascript
+var objA = {prop1: 42};
+var objB = objA; 
+objB.prop1 = 90;
+console.log(objA) 
+```
+The output will `{prop1: 90}` because we're dealing with objects here. Objects are 
+passed by reference, that is, `objA` and `objB` point to the same object in memory. 
+
+### 3. What would be the output of following code?
+
+```javascript
+var objA = {prop1: 42};
+var objB = objA;
+objB = {};
+console.log(objA)
+```
+
+The output will `{prop1: 42}`. 
+
+When we assign `objA` to `objB`, the `objB` variable will point
+to the same object as the `objB` variable.
+
+However, when we reassign `objB` to an empty object, we simply change where `objB` variable references to.
+This doesn't affect where `objA` variable references to. 
+
+### 4. What would be the output of following code?
+
+```javascript
+var arrA = [0,1,2,3,4,5];
+var arrB = arrA;
+arrB[0]=42;
+console.log(arrA)
+```
+
+The output will be `[42,1,2,3,4,5]`. 
+
+Arrays are object in JavaScript and they are passed and assigned by reference. This is why
+both `arrA` and `arrB` point to the same array `[0,1,2,3,4,5]`. That's why changing the first
+element of the `arrB` will also modify `arrA`: it's the same array in the memory.
+
+### 5. What would be the output of following code?
+```javascript
+var arrA = [0,1,2,3,4,5];
+var arrB = arrA.slice();
+arrB[0]=42;
+console.log(arrA)
+```
+
+The output will be `[0,1,2,3,4,5]`. 
+
+The `slice` function copies all the elements of the array returning the new array. That's why
+`arrA` and `arrB` reference two completely different arrays. 
+
+### 5. What would be the output of following code?
+
+```javascript
+var arrA = [{prop1: "value of array A!!"},  {someProp: "also value of array A!"}, 3,4,5];
+var arrB = arrA;
+arrB[0].prop1=42;
+console.log(arrA);
+```
+The output will be `[{prop1: 42},  {someProp: "also value of array A!"}, 3,4,5]`. 
+
+Arrays are object in JS, so both varaibles arrA and arrB point to the same array. Changing
+`arrB[0]` is the same as changing `arrA[0]`
+
+
+### 6. What would be the output of following code?
+
+```javascript
+var arrA = [{prop1: "value of array A!!"},  {someProp: "also value of array A!"}];
+var arrB = arrA.slice();
+arrB[0].prop1=42;
+console.log(arrA);
+```
+
+The output will be `[{prop1: 42},  {someProp: "also value of array A!"}, 3,4,5]`. 
+
+The `slice` function copies all the elements of the array returning the new array. However,
+it doesn't do deep copying. Instead it does shallow copying. You can imagine slice implemented like this: 
+ 
+ ```javascript
+function slice(arr) {
+    var result = [];
+    for (i = 0; i< arr.length; i++) {
+        result.push(arr[i]);
+    }
+    return result; 
+}
+```
+
+Look at the line with `result.push(arr[i])`. If `arr[i]` happens to be a number or string, 
+it will be passed by value, in other words, copied. If `arr[i]` is an object, it will be passed by reference. 
+
+In case of our array `arr[0]` is an object `{prop1: "value of array A!!"}`. Only the reference
+to this object will be copied. This effectively means that arrays arrA and arrB share first
+two elements. 
+
+This is why changing the property of `arrB[0]` in `arrB` will also change the `arrA[0]`.
+
+
 ## Hoisting 
 
 ### 1. console.log(employeeId);
@@ -2088,7 +2207,7 @@ Answer: 1) undefined
 
 Answer: 3) function function
 
-## Object
+## Objects
 
 ### 1. What would be the output of following code ?
 
@@ -2328,7 +2447,7 @@ Answer: 3) foo foo
 	
 Answer: 2) undefined undefined 
 
-## Array
+## Arrays
 
 ### 1. What would be the output of following code?
 
@@ -2598,7 +2717,7 @@ Answer: 1.  [ 'bar', 'john' ] [] [ 'foo' ]
 
 Answer: 3.  [ 15, 16, 2, 23, 42, 8 ]
 
-## Function: 
+## Functions
 
 ### 1. What would be the output of following code ?
 
@@ -3047,7 +3166,8 @@ Answer: 1) Hello John
 
 ### 6. Suggest your question!
 
-## Callback Function
+
+## Callback Functions
 
 ### 1. What would be the output of following code ?
 

From 6aeeb8c23dca9276702291c5ed1a351318369626 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Fri, 28 Sep 2018 14:28:58 +0200
Subject: [PATCH 11/46] style

---
 README.md | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/README.md b/README.md
index 5dd7f75..69837e3 100644
--- a/README.md
+++ b/README.md
@@ -180,7 +180,7 @@ arrayList = [];
 
 The code above will set the variable `arrayList` to a new empty array. This is recommended if you don't have **references to the original array** `arrayList` anywhere else because It will actually create a new empty array. You should be careful with this way of empty the array, because if you have referenced this array from another variable, then the original reference array will remain unchanged, Only use this way if you have only referenced the array by its original variable `arrayList`.
 
-For Instance:
+For instance:
 
 ```javascript
 var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
@@ -197,7 +197,7 @@ arrayList.length = 0;
 
 The code above will clear the existing array by setting its length to 0. This way of emptying an array will also update all the reference variables that point to the original array. 
 
-For Instance:
+For instance:
 
 ```javascript
 var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
@@ -1257,11 +1257,11 @@ for (var i = 0; i < arr.length; i++) {
 
 
 
-## Question 36. What is best way to detect an arrays object on JavaScript ?
+## Question 36. How to check if the value of a variable in an array?
 
 We always encounter in such situation where we need to know whether value is type of array or not.
 
-For Instance : Below code perform some operation based value type
+For instance : the code below perform some operation based value type
 
 ```javascript
 function(value){
@@ -1314,7 +1314,7 @@ So this method is not a best way to detect an array as you can see it's not hand
 
 ECMAScript 5 has introduced **Array.isArray()** method to detect an array type value. The sole purpose of this method is accurately detecting whether a value is an array or not.
 
-In many JavaScript libraries you may see below code for detecting an value of type array.
+In many JavaScript libraries you may see the code below for detecting an value of type array.
 
 ```javascript
 function(value){
@@ -1375,7 +1375,7 @@ console.log(emp1 instanceof Object); // true
 
 The ECMAScript 5 **Object.create()** method is the easiest way for one object to inherit from another. 
 
-**For Instance:** 
+**For instance:** 
 
 ```javascript
 var employee = {
@@ -1391,7 +1391,7 @@ console.log(emp1.displayName());  // output "Nishant"
 
 Above example create a new object `emp1` that inherits from `employee`. Here the inheritance occur as `emp1`'s prototype is set to `employee`. After this emp1 is able to access the same properties and method on employee until new properties or method with the same name are defined.
 
-**For Instance:** Defining `displayName()` method on `emp1` will not automatically override the employee `displayName`.
+**For instance:** Defining `displayName()` method on `emp1` will not automatically override the employee `displayName`.
 
 ```javascript
 emp1.displayName = function() {
@@ -1703,7 +1703,7 @@ If your answer is yes then go and create named function rather anonymous functio
 
 ## Question 47 . How to set a default parameter value ?
 
- If you are coming from python/c# you might be using default value for function parameter incase value(formal parameter) has not been passed. For Instance : 
+ If you are coming from python/c# you might be using default value for function parameter incase value(formal parameter) has not been passed. For instance : 
 
 ```python
 // Define sentEmail function 
@@ -1716,7 +1716,7 @@ def sentEmail(configuration, provider = 'Gmail'):
 
 There are a lot of ways by which you can achieve this in pre ES2015.
 
-Let's understand below code by which we achieved setting default parameter value.
+Let's understand the code below by which we achieved setting default parameter value.
 
 **Method 1: Setting default parameter value** 
 

From 219bfb8afa488f05f44967158628a7fe5be1e200 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sat, 29 Sep 2018 00:25:17 +0200
Subject: [PATCH 12/46] style

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 69837e3..7fe2b3c 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@
 
 It's a book about frontend interview question. We hope that it will help all javascript developers to prepare for a technical job interview. 
 
-## If you want to buy a book in paper form
+## Want to by a book in paper form? Want some badass flashcards?
 
  - This Book will be completed by September 2018 and then it will be available to buy. If you want me to sent an early copy of this book, please add your name and email address in google form here [Google Form](https://goo.gl/forms/c8ubV1tWBBdz6fJP2).
  - If you don't want to wait, you can buy [Yuri's JavaScript Flashcards](http://flashcardsjs.com), a set of frontend interview questions sorted by popularity among the interviewers printed on beautiful poker-size flashcards.

From 02913369035844ecfd03bc8162715b6894b8df76 Mon Sep 17 00:00:00 2001
From: Islam Sayed <islam.sayed8@gmail.com>
Date: Sun, 30 Sep 2018 07:52:52 +0200
Subject: [PATCH 13/46] fix typo

fix typo from by to buy in line 7
---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 7fe2b3c..8f0a937 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@
 
 It's a book about frontend interview question. We hope that it will help all javascript developers to prepare for a technical job interview. 
 
-## Want to by a book in paper form? Want some badass flashcards?
+## Want to buy a book in paper form? Want some badass flashcards?
 
  - This Book will be completed by September 2018 and then it will be available to buy. If you want me to sent an early copy of this book, please add your name and email address in google form here [Google Form](https://goo.gl/forms/c8ubV1tWBBdz6fJP2).
  - If you don't want to wait, you can buy [Yuri's JavaScript Flashcards](http://flashcardsjs.com), a set of frontend interview questions sorted by popularity among the interviewers printed on beautiful poker-size flashcards.

From 5a01d5649995064f0d7794ff6bd1d158dcda1ae9 Mon Sep 17 00:00:00 2001
From: Islam Sayed <islam.sayed8@gmail.com>
Date: Sun, 30 Sep 2018 08:22:39 +0200
Subject: [PATCH 14/46] add output for Boolean + Boolean

Add missing output for the plus operator with all Boolean values which would be "Addition"
---
 README.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/README.md b/README.md
index 7fe2b3c..bae1921 100644
--- a/README.md
+++ b/README.md
@@ -407,10 +407,12 @@ console.log(bar + false);
 The code above will output `1, "truexyz", 2, 1` as output. Here's a general guideline  for the plus operator:
   - Number + Number  -> Addition
   - Boolean + Number -> Addition
+  - Boolean + Boolean -> Addition
   - Number + String  -> Concatenation
   - String + Boolean -> Concatenation
   - String + String  -> Concatenation
   
+  
 
 ## Question 15. What will be the output of the following code?
 

From 5aaf80884a9bce5414c21d13a696513af8a060a2 Mon Sep 17 00:00:00 2001
From: Umesh Kumar <creator1134@users.noreply.github.com>
Date: Mon, 1 Oct 2018 18:04:12 +0530
Subject: [PATCH 15/46] fix typo error

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 4de35de..14d0190 100644
--- a/README.md
+++ b/README.md
@@ -837,7 +837,7 @@ obj1.myMethod(); // will print "Hi there" following with obj1.
 ## Question 26. What is IIFE (Immediately Invoked Function Expression) and how it can be useful?
 ### Answer 
 #### Definition
-IIFE a function that runs as soon as it's defined. Usually it's anonymous (doesn't have a function name), but it also can be named. Here's an exmaple of IIFE:
+IIFE a function that runs as soon as it's defined. Usually it's anonymous (doesn't have a function name), but it also can be named. Here's an example of IIFE:
 
 ```javascript
 (function() {

From 8efd04a2d5e0d4ee957bb00bc065f16c8d1d4f2f Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sat, 6 Oct 2018 17:17:30 +0200
Subject: [PATCH 16/46] terminological changes

---
 README.md | 50 ++++++++++++++++++++++++--------------------------
 1 file changed, 24 insertions(+), 26 deletions(-)

diff --git a/README.md b/README.md
index 14d0190..2587739 100644
--- a/README.md
+++ b/README.md
@@ -477,13 +477,13 @@ var foo = function bar() {
 ```javascript
 var foo = function() {
   // Some code
-};
+}
 ```
 
 ```javascript
 function bar () {
   // Some code
-};
+}
 ```
 ### Answer 
 
@@ -502,21 +502,23 @@ The main difference is function `foo` is defined at `run-time` and is called fun
 bar(); // Call bar function here, It will not give an Error
 function bar() {
   console.log("Hi I am inside Foo");
-};
+}
 ```
 
-## Question 18. what is function hoisting in JavaScript?
-**Function Expression**
+## Question 18. In which case the function definition is not hoisted in JavaScript?
+
+
+### Answer 
+
+Let's take the following **function expression**
 
 ```javascript
  var foo = function foo() {
      return 12;
- };
+ }
 ```
 
-### Answer 
-
-In JavaScript `var`-declared variables and functions are `hoisted`. Let's take function `hoisting` first. Basically, the JavaScript interpreter looks ahead to find all the variable declaration and hoists them to the top of the function where it's declared. For Example:
+In JavaScript `var`-declared variables and functions are `hoisted`. Let's take function `hoisting` first. Basically, the JavaScript interpreter looks ahead to find all the variable declaration and hoists them to the top of the function where it's declared. For example:
 
 ```javascript
 foo(); // Here foo is still undefined
@@ -525,7 +527,7 @@ var foo = function foo() {
 };
 ```
 
-The code above behind the scene look something like below code:
+The code above behind the scene look something like this:
 
 ```javascript
 var foo = undefined;
@@ -1371,11 +1373,9 @@ console.log(emp1 instanceof Employee); // true
 console.log(emp1 instanceof Object); // true
 ```
 
-## Question 38. Describe Object-Based inheritance in JavaScript.
-
- Object-based inheritance also called prototypal inheritance in which we one object inherit from another object without invoking a constructor function.
+## Question 38. How does Object.create method works JavaScript?
 
-The ECMAScript 5 **Object.create()** method is the easiest way for one object to inherit from another. 
+The ECMAScript 5 **Object.create()** method is the easiest way for one object to inherit from another, without invoking a constructor function. 
 
 **For instance:** 
 
@@ -1391,7 +1391,7 @@ var emp1 = Object.create(employee);
 console.log(emp1.displayName());  // output "Nishant"
 ```
 
-Above example create a new object `emp1` that inherits from `employee`. Here the inheritance occur as `emp1`'s prototype is set to `employee`. After this emp1 is able to access the same properties and method on employee until new properties or method with the same name are defined.
+In the example above, we create a new object `emp1` that inherits from `employee`. In other words `emp1`'s prototype is set to `employee`. After this emp1 is able to access the same properties and method on employee until new properties or method with the same name are defined.
 
 **For instance:** Defining `displayName()` method on `emp1` will not automatically override the employee `displayName`.
 
@@ -1406,7 +1406,7 @@ emp1.displayName();//xyz-Anonymous
 
 In addition to this **`Object.create(`)** method also allows to specify a second argument which is an object containing additional properties and methods to add to the new object.
 
-**For Example**
+**For example**
 
 ```javascript
 var emp1 = Object.create(employee, {
@@ -1418,13 +1418,11 @@ var emp1 = Object.create(employee, {
 emp1.displayName(); // "John"
 employee.displayName(); // "Nishant"
 ```
-In above example, `emp1` is created with it's own value for name, so calling **displayName()** method display `"John"` instead of `"Nishant"`.
+In the example above, `emp1` is created with it's own value for name, so calling **displayName()** method will display `"John"` instead of `"Nishant"`.
 
 Object created in this manner give you full control over newly created object. You are free to add, remove any properties and method you want.
 
-## Question 39. Describe Type-Based inheritance in JavaScript.
-
-Type-based inheritance works with constructor function instead of object, It means we need to call constructor function of the object from which you  want to inherit.
+## Question 39. How to use constructor functions for inheritance in JavaScript?
 
 Let say we have `Person` class which has name, age, salary properties and **incrementSalary()** method.
 
@@ -1449,7 +1447,7 @@ function Employee(company){
 //Prototypal Inheritance 
 Employee.prototype = new Person("Nishant", 24,5000);
 ```
-In above example, **Employee** type inherits from **Person**, which is called super types. It does so by assigning a new instance of Person to Employee prototype. After that, every instance of Employee inherits it's properties and methods from Person.
+In the example above, **Employee** type inherits from **Person**. It does so by assigning a new instance of `Person` to `Employee` prototype. After that, every instance of `Employee` inherits its properties and methods from `Person`.
 
 ```javascript
 //Prototypal Inheritance 
@@ -1493,7 +1491,7 @@ There are three levels of preventing modification:
 
 No new properties or methods can be added to the object, but one can change the existing properties and method.
 
-For Example: 
+For example: 
 
 ```javascript
 var employee = {
@@ -1567,7 +1565,7 @@ Frozen objects are considered both non-extensible and sealed.
 
 If you are decided to prevent modification, sealed, freeze the object then use in strict mode so that you can catch the error.
 
-For Example: 
+For example: 
 
 ```javascript
 "use strict";
@@ -1791,7 +1789,7 @@ name , age , addressLine1 , addressLine2 , city */
 const merge = (toObj, fromObj) = Object.assign(toObj, fromObj);
 ```
  
-**Method 2: Without using in-built function**
+**Method 2: Without using built-in function**
 
 ```javascript
 function merge(toObj, fromObj) {
@@ -1809,7 +1807,7 @@ function merge(toObj, fromObj) {
   }
 }
 ```
-## Question 49. What is non-enumerable property in JavaScript and how can create ?
+## Question 49. What is non-enumerable property in JavaScript and how you can create one?
 
 Object can have properties that don't show up when you iterate through object using for...in loop or using Object.keys() to get an array of property names. This properties is know as non-enumerable properties.
 
@@ -1843,7 +1841,7 @@ Object.defineProperty(person, 'phoneNo',{
 
 Object.keys(person); // ['name', 'salary', 'country']
 ```
-In above example `phoneNo` property didn't show up because we made it non-enumerable by setting **enumerable:false**
+In the example above `phoneNo` property didn't show up because we made it non-enumerable by setting **enumerable:false**
 
 Now let's try to change value of `phoneNo`
 

From aafa07a441f30ad72f249691a0741855253d5cbf Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sat, 6 Oct 2018 17:35:57 +0200
Subject: [PATCH 17/46] Question 3 clearer examples

---
 README.md | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/README.md b/README.md
index 2587739..f4d2b03 100644
--- a/README.md
+++ b/README.md
@@ -59,11 +59,11 @@ The same holds true for any value of x that being converted to Number, returns N
 
 This is why you need to pay attention when you deal with numeric variables. `NaN` can’t be equal, less than or more than any other numeric value, so the only reliable way to check if the value is `NaN`, is to use `isNaN()` function.
 
-## Question 3. What is the drawback of having private methods in a JavaScript object?
+## Question 3. What is the drawback of declaring methods directly in JavaScript objects?
 
 ### Answer 
 
-One of the drawback of creating a private method in JavaScript is that they are very memory inefficient because a new copy of the method would be created for each instance.
+One of the drawback of declaring methods directly in JavaScript objects is that they are very memory inefficient.  When you do that, a new copy of the method is created for each instance of an object. Let's see it on example:
 
 ```javascript
 var Employee = function (name, company, salary) {
@@ -71,18 +71,24 @@ var Employee = function (name, company, salary) {
   this.company = company || "";
   this.salary = salary || 5000;
 
-  // We can create a private method like this
-  var increaseSalary = function () {
-    this.salary = this.salary + 1000;
+  // We can create a method like this:
+  this.formatSalary = function () {
+      return "$ " + this.salary;
   };
-
 };
 
-```
+// we can also create method in Employee's prototype:
+Employee.prototype.formatSalary2 = function() {
+    return "$ " + this.salary;
+}
 
-Here each instance variable `emp1`, `emp2`, `emp3` has own copy of increaseSalary private method. However the `displayIncreasedSalary` will only be added once to an object `Employee.prototype`.
+//creating objects
+var emp1 = new Employee('Yuri Garagin', 'Company 1', 1000000);
+var emp2 = new Employee('Dinesh Gupta', 'Company 2', 1039999);
+var emp3 = new Employee('Erich Fromm', 'Company 3', 1299483);
+```
 
-So as recommendation don't go for a private method unless it's necessary.
+Here each instance variable `emp1`, `emp2`, `emp3` has own copy of `formatSalary` method. However the `formatSalary2` will only be added once to an object `Employee.prototype`.
 
 ## Question 4. What is “closure” in javascript? Can you provide an example?
 

From d3ae6bddd2b2c8cb530c0174e59c6d89175138d6 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sun, 11 Nov 2018 19:19:25 +0100
Subject: [PATCH 18/46] #46 fixed thanks to @nomean42

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index f4d2b03..037cb10 100644
--- a/README.md
+++ b/README.md
@@ -2776,7 +2776,7 @@ Answer: 4) undefined true
 var obj = {
 	message: "Hello",
 	innerMessage: function() {
-		console.log(this.message);
+		return this.message;
 	}
 };
 	

From 3d27d2d6f099458dcbfefbb962c43c99552a20e3 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sun, 11 Nov 2018 19:20:23 +0100
Subject: [PATCH 19/46] #63 fixed thanks to @nomean42

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 037cb10..fbfee0e 100644
--- a/README.md
+++ b/README.md
@@ -2487,7 +2487,7 @@ Answer: 3) ["100"] 1
 ```
 
 1.  [] [] [Array[5]] 1
-2.  [] [undefined × 100] Array[5] 5 
+2.  [] [undefined × 100] Array[5] 1
 3.  [] [] ['1',2,'3',4,5.6] 5
 4.  [] [] [Array[5]] 5 
 

From c1022254dbfac53514ad533a0c188fac572d6e15 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sun, 11 Nov 2018 19:21:11 +0100
Subject: [PATCH 20/46] #61 fixed thanks to @nomean42

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index fbfee0e..58d6be3 100644
--- a/README.md
+++ b/README.md
@@ -1792,7 +1792,7 @@ name , age , addressLine1 , addressLine2 , city */
 **Method 1: Using ES6, Object assign method**
 
 ```javascript
-const merge = (toObj, fromObj) = Object.assign(toObj, fromObj);
+const merge = (toObj, fromObj) => Object.assign(toObj, fromObj);
 ```
  
 **Method 2: Without using built-in function**

From f0eedd61a53117cfddfdcb5cff02b1ae04a5ce3e Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sun, 11 Nov 2018 19:24:41 +0100
Subject: [PATCH 21/46] #61 fixed thanks to @nomean42

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 58d6be3..cc71d94 100644
--- a/README.md
+++ b/README.md
@@ -1789,7 +1789,7 @@ merge(person , address);
 /* Now person should have 5 properties 
 name , age , addressLine1 , addressLine2 , city */
 ```
-**Method 1: Using ES6, Object assign method**
+**Method 1: Using ES6, Object.assign method**
 
 ```javascript
 const merge = (toObj, fromObj) => Object.assign(toObj, fromObj);

From cdb86b673e78b6dd065e4cc550ecb7b1ab872820 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sun, 11 Nov 2018 19:27:31 +0100
Subject: [PATCH 22/46] #60 fixed thanks to @vibl

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index cc71d94..a7c6b85 100644
--- a/README.md
+++ b/README.md
@@ -1618,7 +1618,7 @@ function appLog() {
   console.log.apply(console, args);
 }
 
-console.log(appLog("Some error message")); 
+appLog("Some error message"); 
 //output of above console: 'your app name Some error message'
 ```
 

From 561f8f17df47ff52116634a1510673e3286627e4 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Sun, 11 Nov 2018 19:29:38 +0100
Subject: [PATCH 23/46] #59 fixed thanks to @vibl

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index a7c6b85..391a54b 100644
--- a/README.md
+++ b/README.md
@@ -1091,7 +1091,7 @@ Solution:
 function deepClone(object){
 	var newObject = {};
 	for(var key in object){
-		if(typeof object[key] === 'object'){
+		if(typeof object[key] === 'object'  && object[key] !== null ){
 		 newObject[key] = deepClone(object[key]);
 		}else{
 		 newObject[key] = object[key];

From 607d5620959720df7970c9d8db5b0bb6e48f9894 Mon Sep 17 00:00:00 2001
From: Orinami Olatunji <orinamiolatunji@gmail.com>
Date: Sat, 24 Nov 2018 14:40:40 +0100
Subject: [PATCH 24/46] Update README.md

Fixed what I think is a typo in line 16:

'will stop execute thereafter' to 'will stop executing thereafter'
---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 391a54b..4d35cb3 100644
--- a/README.md
+++ b/README.md
@@ -13,7 +13,7 @@ It's a book about frontend interview question. We hope that it will help all jav
 
 ### Answer
 
-In JavaScript if you try to use a variable that doesn't exist and has not been declared, then JavaScript will throw an error `var name is not defined` and the script will stop execute thereafter. But If you use `typeof undeclared_variable` then it will return `undefined`.
+In JavaScript if you try to use a variable that doesn't exist and has not been declared, then JavaScript will throw an error `var name is not defined` and the script will stop executing thereafter. But If you use `typeof undeclared_variable` then it will return `undefined`.
 
 Before starting further discussion let's understand the difference between declaration and definition.
 

From 18fb7ca907d483c41f94f171ca13f68eef601af8 Mon Sep 17 00:00:00 2001
From: LLeeL <47989994+LLeeL@users.noreply.github.com>
Date: Wed, 20 Mar 2019 22:18:15 -0400
Subject: [PATCH 25/46] Change coding questions 6

missing 3,4,5 in array (otherwise will not match the output)
 add  arrB[3] = 20; to show the difference between passed by value and passed by reference
---
 README.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 4d35cb3..03ca4aa 100644
--- a/README.md
+++ b/README.md
@@ -1992,9 +1992,10 @@ Arrays are object in JS, so both varaibles arrA and arrB point to the same array
 ### 6. What would be the output of following code?
 
 ```javascript
-var arrA = [{prop1: "value of array A!!"},  {someProp: "also value of array A!"}];
+var arrA = [{prop1: "value of array A!!"}, {someProp: "also value of array A!"},3,4,5];
 var arrB = arrA.slice();
 arrB[0].prop1=42;
+arrB[3] = 20;
 console.log(arrA);
 ```
 

From b461d61f33c3f204e5a76d69689b7e1f4db7838d Mon Sep 17 00:00:00 2001
From: Anand Shenoy <anand.shenoy14@gmail.com>
Date: Mon, 13 May 2019 21:30:54 -0700
Subject: [PATCH 26/46] Adding a Question between 17 and 18

---
 README.md | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 03ca4aa..8e7a9c1 100644
--- a/README.md
+++ b/README.md
@@ -478,7 +478,7 @@ var foo = function bar() {
 // bar is undefined here
 ```
 
-## Question 17. What is the difference between declaring a function in the formats listed below?
+## Question 17a. What is the difference between declaring a function in the formats listed below?
 
 ```javascript
 var foo = function() {
@@ -510,6 +510,21 @@ function bar() {
   console.log("Hi I am inside Foo");
 }
 ```
+## Question 17b. What is the output of the following?
+
+```javascript
+bar();
+(function abc(){console.log('something')})();
+function bar(){console.log('bar got called')};
+```
+### Answer 
+
+The output will be :
+``` 
+bar got called
+something
+```
+Since the function is called first and defined during parse time the JS engine will try to find any possible parse time definitions and start the execution loop which will mean function is called first even if the definition is post another function.
 
 ## Question 18. In which case the function definition is not hoisted in JavaScript?
 

From c49bc46a18488957e0f63097769726f36d5d5fb4 Mon Sep 17 00:00:00 2001
From: Chris Diana <cdiana.media@gmail.com>
Date: Sat, 1 Jun 2019 11:10:06 -0600
Subject: [PATCH 27/46] Function declaration typo and grammar fixes

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 03ca4aa..48eb6b8 100644
--- a/README.md
+++ b/README.md
@@ -493,7 +493,7 @@ function bar () {
 ```
 ### Answer 
 
-The main difference is function `foo` is defined at `run-time` and is called function expression, whereas function `bar` is defined at parse time and is called function statement. To understand in better, let's see below code :
+The main difference is that function `foo` is defined at `run-time` and is called a function expression, whereas function `bar` is defined at `parse time` and is called a function statement. To understand it better, let's take a look at the code below :
 
 ```javascript
 // Run-Time function declaration

From c04040bdc98c34245cfdec21c3a6bdde4e98d999 Mon Sep 17 00:00:00 2001
From: Yuri Katkov <katkov.juriy@gmail.com>
Date: Mon, 8 Jul 2019 17:01:48 +0300
Subject: [PATCH 28/46] Update README.md

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index edaa43b..f27ba64 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@ It's a book about frontend interview question. We hope that it will help all jav
 
 ## Want to buy a book in paper form? Want some badass flashcards?
 
- - This Book will be completed by September 2018 and then it will be available to buy. If you want me to sent an early copy of this book, please add your name and email address in google form here [Google Form](https://goo.gl/forms/c8ubV1tWBBdz6fJP2).
+ - This Book will be soon completed and then it will be available to buy in a paper form. If you want me to sent an early copy of this book, please add your name and email address in google form here [Google Form](https://goo.gl/forms/c8ubV1tWBBdz6fJP2).
  - If you don't want to wait, you can buy [Yuri's JavaScript Flashcards](http://flashcardsjs.com), a set of frontend interview questions sorted by popularity among the interviewers printed on beautiful poker-size flashcards.
 
 ## Question 1. What's the difference between `undefined` and `not defined` in JavaScript

From f4dcf2921fccdd86bc4e20f25820f4f9031b14da Mon Sep 17 00:00:00 2001
From: Vova <vboretskyi@gmail.com>
Date: Fri, 27 Sep 2019 21:50:44 +0300
Subject: [PATCH 29/46] resolve
 ganqqwerty/123-Essential-JavaScript-Interview-Questions#78

---
 README.md | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index f27ba64..74ecb19 100644
--- a/README.md
+++ b/README.md
@@ -1864,16 +1864,15 @@ Object.keys(person); // ['name', 'salary', 'country']
 ```
 In the example above `phoneNo` property didn't show up because we made it non-enumerable by setting **enumerable:false**
 
+**Bonus**
+
 Now let's try to change value of `phoneNo`
 
 ```javascript
 person.phoneNo = '7777777777'; 
 ```
-Changing non-enumerable property value will return error in `strict mode`. In non-strict mode it won't through any error but it won't change the value of phoneNo.
-
-**Bonus**
 
-**Object.defineProperty()** is also let you create read-only properties as we saw above, we are not able to modify phoneNo value of a person object.
+**Object.defineProperty()** also lets you create read-only properties as we saw above, we are not able to modify phoneNo value of a person object. This is because descriptor has **writable** property, which is `false` by default. Changing non-writable property value will return error in strict mode. In non-strict mode it won't through any error but it won't change the value of phoneNo.
 
 ## Question 50. What is Function binding ?
 

From b7c226654c95d98054d2330e01b9c484ed467678 Mon Sep 17 00:00:00 2001
From: Pramod Mali <pramodmaali@gmail.com>
Date: Tue, 5 Nov 2019 21:23:39 +0530
Subject: [PATCH 30/46] Added Interview Question

---
 README.md | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/README.md b/README.md
index 74ecb19..d0adc70 100644
--- a/README.md
+++ b/README.md
@@ -3391,7 +3391,32 @@ console.log(mul(2)(3)(4)(5)(6));
 	
 Answer: 1) 720
 
+### 6. What would be the output of following code ?
+
+```javascript
+function getName1(){
+	console.log(this.name);
+}
+
+Object.prototype.getName2 = () =>{
+	console.log(this.name)
+}
+
+let personObj = {
+	name:"Tony",
+	print:getName1
+}
+
+personObj.print();
+personObj.getName2();
+```
+
+1. undefined undefined
+2. Tony undefined
+3. undefined Tony
+4. Tony Tony
 
+Answer: 2) Tony undefined
 
 
 

From 3c6405cc89d3a795ee7a9a6bb0ee08996862d9a2 Mon Sep 17 00:00:00 2001
From: Pramod Mali <pramodmaali@gmail.com>
Date: Tue, 5 Nov 2019 23:15:29 +0530
Subject: [PATCH 31/46] Update README.md

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index d0adc70..5737ad9 100644
--- a/README.md
+++ b/README.md
@@ -3391,7 +3391,7 @@ console.log(mul(2)(3)(4)(5)(6));
 	
 Answer: 1) 720
 
-### 6. What would be the output of following code ?
+### 7. What would be the output of following code ?
 
 ```javascript
 function getName1(){

From 616695c43737d82e2767a8fd7d057deba5c9bc38 Mon Sep 17 00:00:00 2001
From: Tomer Ovadia <o.tomerr@gmail.com>
Date: Tue, 12 Nov 2019 19:38:42 +0200
Subject: [PATCH 32/46] Update README.md

---
 README.md | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/README.md b/README.md
index 74ecb19..eeb3953 100644
--- a/README.md
+++ b/README.md
@@ -1352,6 +1352,18 @@ function(value){
 }
 ```
 
+**Method 4:**
+
+You can query the constructor name:
+
+```javascript
+function isArray(value) {
+	return value.constructor.name === "Array";
+}
+
+```
+
+
 ## Question 37. Best way to detect reference values of any type in JavaScript ?
 
  In Javascript Object are called as reference type, Any value other then primitive is definitely a reference type. There are several built-in reference type such as **Object**, **Array**, **Function**, **Date**, **null** and **Error**.

From 234c12998b88e831cf4711649d09f9d43e6b0911 Mon Sep 17 00:00:00 2001
From: Tomer Ovadia <o.tomerr@gmail.com>
Date: Tue, 12 Nov 2019 19:45:04 +0200
Subject: [PATCH 33/46] Update README.md

---
 README.md | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/README.md b/README.md
index eeb3953..c638e88 100644
--- a/README.md
+++ b/README.md
@@ -1363,6 +1363,15 @@ function isArray(value) {
 
 ```
 
+**Method 5:**
+
+You check  if a given value is an `instanceof Array`:
+
+```javascript
+function isArray(value) {
+	return value instanceof Array;
+}
+```
 
 ## Question 37. Best way to detect reference values of any type in JavaScript ?
 

From a294916d3d74db80457150c9c65b7855131c521a Mon Sep 17 00:00:00 2001
From: Harman Kumar <50094184+harmansingha@users.noreply.github.com>
Date: Sat, 30 Nov 2019 12:53:33 +0530
Subject: [PATCH 34/46] Added New Method for setting default value in ES6

---
 README.md | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/README.md b/README.md
index c638e88..6124331 100644
--- a/README.md
+++ b/README.md
@@ -1802,6 +1802,27 @@ sentEmail({
 }, 'Yahoo Mail');
 ```
 
+**Method 3: Setting default parameter value in ES6**
+```javascript
+function sendEmail(configuration, provider = "Gmail") {
+  // Set default value if user has not passed value for provider
+  
+  // Value of provider can be accessed directly
+  console.log(`Provider: ${provider}`);
+}
+
+// In this call we are not passing provider parameter value
+sentEmail({
+  from: 'xyz@gmail.com',
+  subject: 'Test Email'
+});
+// Here we are passing Yahoo Mail as a provider value
+sentEmail({
+  from: 'xyz@gmail.com',
+  subject: 'Test Email'
+}, 'Yahoo Mail');
+```
+
 ## Question 48. Write code for merge two JavaScript Object dynamically.
 Let say you have two objects 
 

From 3f0a94f412d962b149677a35bbd9f70df92734e4 Mon Sep 17 00:00:00 2001
From: Moshiur Rahman <15215020+moshiurse@users.noreply.github.com>
Date: Sun, 1 Dec 2019 00:06:28 +0600
Subject: [PATCH 35/46] fix duplicate variable name

---
 README.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index c638e88..4c4b870 100644
--- a/README.md
+++ b/README.md
@@ -1046,8 +1046,8 @@ This method is useful if we want to create several similar objects. In the code
 
   // Creating multiple object which have similar property but diff value assigned to object property.
   var employee1 = new Employee('John', 'Moto', 24, '5000$');
-  var employee1 = new Employee('Ryan', 'Jor', 26, '3000$');
-  var employee1 = new Employee('Andre', 'Salt', 26, '4000$');
+  var employee2 = new Employee('Ryan', 'Jor', 26, '3000$');
+  var employee3 = new Employee('Andre', 'Salt', 26, '4000$');
 ```
 
 #### Method 2: Object Literal

From c6439af1d361b2f896909dc883e8de254aca60e7 Mon Sep 17 00:00:00 2001
From: Pramod Mali <pramodmaali@gmail.com>
Date: Tue, 3 Dec 2019 17:21:45 +0530
Subject: [PATCH 36/46] Added explaination to Question

---
 README.md | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/README.md b/README.md
index 5737ad9..746f472 100644
--- a/README.md
+++ b/README.md
@@ -3418,7 +3418,26 @@ personObj.getName2();
 
 Answer: 2) Tony undefined
 
+Explaination: **getName1()** function works fine because it's being called from ***personObj***, so it has access to *this.name* property. But when while calling **getnName2** which is defined under *Object.prototype* doesn't have any proprty named *this.name*. There should be *name* property under prototype. Following is the code:
 
+```javascript
+function getName1(){
+	console.log(this.name);
+}
+
+Object.prototype.getName2 = () =>{
+  console.log(Object.getPrototypeOf(this).name);
+}
+
+let personObj = {
+	name:"Tony",
+	print:getName1
+}
+
+personObj.print();
+Object.prototype.name="Steve";
+personObj.getName2();
+```
 
 
 ## Contributing

From cb6acce9a22d7d2ff3417e07418c4a73e394ff0f Mon Sep 17 00:00:00 2001
From: Pramod Mali <pramodmaali@gmail.com>
Date: Mon, 13 Jan 2020 23:01:16 +0530
Subject: [PATCH 37/46] Updating question 35

---
 README.md | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/README.md b/README.md
index 746f472..bb1afb0 100644
--- a/README.md
+++ b/README.md
@@ -1280,7 +1280,16 @@ for (var i = 0; i < arr.length; i++) {
 }
 ```
 
+This can also achieve by forEach (allows you to keep that variable within the forEach’s scope)
 
+```javascript 
+var arr = [10, 32, 65, 2];
+arr.forEach(function(i) {
+  setTimeout(function() {
+    console.log('The index of this number is: ' + i);
+  }, 3000);
+})
+```
 
 ## Question 36. How to check if the value of a variable in an array?
 

From ff5be2a8fc4e76fc3204b0996403c6f34e2ba6d2 Mon Sep 17 00:00:00 2001
From: Rahul Kumar <37505384+rahuldbs@users.noreply.github.com>
Date: Mon, 6 Apr 2020 21:54:11 +0530
Subject: [PATCH 38/46] corrected the foreach solution of question 35

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 5e0d5f2..c222fdd 100644
--- a/README.md
+++ b/README.md
@@ -1284,7 +1284,7 @@ This can also achieve by forEach (allows you to keep that variable within the fo
 
 ```javascript 
 var arr = [10, 32, 65, 2];
-arr.forEach(function(i) {
+arr.forEach(function(ele, i) {
   setTimeout(function() {
     console.log('The index of this number is: ' + i);
   }, 3000);

From fae6fabf9a32eb6bf0e656189dcef47e038f32d3 Mon Sep 17 00:00:00 2001
From: thinhnv <maxiqboy@gmail.com>
Date: Sun, 9 Aug 2020 16:38:42 +0700
Subject: [PATCH 39/46] :art: [REFACTOR] - Add collapsed answers - fix the
 counting wrong

 1 - Hide the answers in the collapsed section! Make people easy to practise the questions!
 2 - Fix the counting wrong in question 40-44 ( missing 41 42 43 )
---
 README.md | 634 ++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 524 insertions(+), 110 deletions(-)

diff --git a/README.md b/README.md
index c222fdd..5a4ba4d 100644
--- a/README.md
+++ b/README.md
@@ -11,7 +11,7 @@ It's a book about frontend interview question. We hope that it will help all jav
 
 ## Question 1. What's the difference between `undefined` and `not defined` in JavaScript
 
-### Answer
+<details><summary><b>Answer</b></summary>
 
 In JavaScript if you try to use a variable that doesn't exist and has not been declared, then JavaScript will throw an error `var name is not defined` and the script will stop executing thereafter. But If you use `typeof undeclared_variable` then it will return `undefined`.
 
@@ -42,6 +42,7 @@ console.log(y);  // Output: ReferenceError: y is not defined
 ### Ref Link:
 [http://stackoverflow.com/questions/20822022/javascript-variable-definition-declaration](http://stackoverflow.com/questions/20822022/javascript-variable-definition-declaration)
 
+</details>
 
 ## Question 2. For which value of `x` the results of the following statements are not the same?
 
@@ -50,7 +51,7 @@ console.log(y);  // Output: ReferenceError: y is not defined
 //  if( x <= 100 ) {...}
 if( !(x > 100) ) {...}
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 `NaN <= 100` is `false` and `NaN > 100` is also `false`, so if the
 value of `x` is `NaN`, the statements are not the same.
@@ -59,9 +60,11 @@ The same holds true for any value of x that being converted to Number, returns N
 
 This is why you need to pay attention when you deal with numeric variables. `NaN` can’t be equal, less than or more than any other numeric value, so the only reliable way to check if the value is `NaN`, is to use `isNaN()` function.
 
+</details>
+
 ## Question 3. What is the drawback of declaring methods directly in JavaScript objects?
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 One of the drawback of declaring methods directly in JavaScript objects is that they are very memory inefficient.  When you do that, a new copy of the method is created for each instance of an object. Let's see it on example:
 
@@ -90,9 +93,11 @@ var emp3 = new Employee('Erich Fromm', 'Company 3', 1299483);
 
 Here each instance variable `emp1`, `emp2`, `emp3` has own copy of `formatSalary` method. However the `formatSalary2` will only be added once to an object `Employee.prototype`.
 
+</details>
+
 ## Question 4. What is “closure” in javascript? Can you provide an example?
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 A closure is a function defined inside another function (called parent function) and has access to the variable which is declared and defined in parent function scope.
 
@@ -136,13 +141,15 @@ innerFuncVar = y
 globalVar = abc
 ```
 
+</details>
+
 ## Question 5. Write a mul function which will work properly when invoked with following syntax.
 
 ```javascript
 console.log(mul(2)(3)(4)); // output : 24
 console.log(mul(4)(3)(4)); // output : 48
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 Below is the code followed by the explanation of how it works:
 
@@ -165,6 +172,8 @@ In Javascript function defined inside has access to outer function variable and
 - A function can be pass as a parameter to another function
 - A function can be returned from another function
 
+</details>
+
 ## Question 6. How to empty an array in JavaScript?
 For instance:
 
@@ -174,7 +183,7 @@ var arrayList =  ['a', 'b', 'c', 'd', 'e', 'f'];
 
 How can we empty the array above?
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 There are a couple of ways by which we can empty an array, So let's discuss all the possible way by which we can empty an array.
 
@@ -238,9 +247,11 @@ while(arrayList.length) {
 Above implementation can also empty the array. But not recommended to use often.
 
 
+</details>
+
 ## Question 7. How to check if an object is an array or not?
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The best way to find whether an object is instance of a particular class or not using `toString` method from `Object.prototype`
 
@@ -303,6 +314,8 @@ Array.isArray(arrayList);
 `Array.isArray` is supported by Chrome 5, Firefox 4.0, IE 9, Opera 10.5 and Safari 5
 
 
+</details>
+
 ## Question 8. What will be the output of the following code?
 
 ```javascript
@@ -313,11 +326,13 @@ var output = (function(x) {
 
 console.log(output);
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The code above will output `0` as output. `delete` operator is used to delete a property from an object. Here `x` is not an object it's **local variable**. `delete` operator doesn't affect local variables.
 
 
+</details>
+
 ## Question 9. What will be the output of the following code?
 
 ```javascript
@@ -329,11 +344,13 @@ var output = (function() {
 
 console.log(output);
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The code above will output `1` as output. `delete` operator is used to delete a property from an object. Here `x` is not an object it's **global variable** of type `number`.
 
 
+</details>
+
 ## Question 10. What will be the output of the following code?
 
 ```javascript
@@ -345,11 +362,13 @@ var output = (function() {
 
 console.log(output);
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The code above will output `undefined` as output. `delete` operator is used to delete a property from an object. Here `x` is an object which has foo as a property and from a self-invoking function, we are deleting the `foo` property of object `x` and after deletion, we are trying to reference deleted property `foo` which result `undefined`.
 
 
+</details>
+
 ## Question 11. What will be the output of the following code?
 
 ```javascript
@@ -361,12 +380,14 @@ delete emp1.company
 console.log(emp1.company);
 ```
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 The code above will output `xyz` as output. Here `emp1` object got company as **prototype** property. delete operator doesn't delete prototype property.
 
 `emp1` object doesn't have **company** as its own property. you can test it `console.log(emp1.hasOwnProperty('company')); //output : false` However, we can delete company property directly from `Employee` object using `delete Employee.company` or we can also delete from `emp1` object using `__proto__` property `delete emp1.__proto__.company`.
 
 
+</details>
+
 ## Question 12. What is `undefined x 1` in JavaScript
 
 ```javascript
@@ -374,7 +395,7 @@ var trees = ["redwood", "bay", "cedar", "oak", "maple"];
 delete trees[3];
 ```
 
-### Answer 
+<details><summary><b>Answer</b></summary>
  - When you run the code above and do `console.log(trees);` in chrome developer console then you will get `["redwood", "bay", "cedar", undefined × 1, "maple"]`.
  - In the recent versions of Chrome you will see the word `empty` of `undefined x 1`.
  - When you run the same code in Firefox browser console then you will get `["redwood", "bay", "cedar", undefined, "maple"]`
@@ -385,6 +406,8 @@ Clearly we can see that Chrome has its own way of displaying uninitialized index
 
 
 
+</details>
+
 ## Question 13. What will be the output of the following code?
 
 ```javascript
@@ -392,13 +415,15 @@ var trees = ["xyz", "xxxx", "test", "ryan", "apple"];
 delete trees[3];
 console.log(trees.length);
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 The code above will output `5` as output. When we used `delete` operator for deleting an array element then, the array length is not affected by this. This holds even if you deleted all elements of an array using `delete` operator.
 
 So when delete operator removes an array element that deleted element is no longer present in the array. In place of value at deleted index `undefined x 1` in **chrome** and `undefined` is placed at the index. If you do `console.log(trees)` output `["xyz", "xxxx", "test", undefined × 1, "apple"]` in Chrome and in Firefox `["xyz", "xxxx", "test", undefined, "apple"]`.
 
 
 
+</details>
+
 ## Question 14. What will be the output of the following code?
 
 ```javascript
@@ -408,7 +433,7 @@ console.log(bar + "xyz");
 console.log(bar + true);  
 console.log(bar + false);
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The code above will output `1, "truexyz", 2, 1` as output. Here's a general guideline  for the plus operator:
   - Number + Number  -> Addition
@@ -420,13 +445,15 @@ The code above will output `1, "truexyz", 2, 1` as output. Here's a general guid
   
   
 
+</details>
+
 ## Question 15. What will be the output of the following code?
 
 ```javascript
 var z = 1, y = z = typeof y;
 console.log(y);
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The code above will print string `"undefined"` as output. According to associativity rule operator with the same precedence are processed based on their associativity property of operator. Here associativity of the assignment operator is `Right to Left` so first `typeof y` will evaluate first which is string `"undefined"` and assigned to `z` and then `y` would be assigned the value of z. The overall sequence will look like that: 
 
@@ -438,6 +465,8 @@ z = typeof y;
 y = z;
 ```
 
+</details>
+
 ## Question 16. What will be the output of the following code?
 
 ```javascript
@@ -446,7 +475,7 @@ var foo = function bar() { return 12; };
 typeof bar();
 ```
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The output will be `Reference Error`. To fix the bug we can try to rewrite the code a little bit: 
 
@@ -478,6 +507,8 @@ var foo = function bar() {
 // bar is undefined here
 ```
 
+</details>
+
 ## Question 17a. What is the difference between declaring a function in the formats listed below?
 
 ```javascript
@@ -491,7 +522,7 @@ function bar () {
   // Some code
 }
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The main difference is that function `foo` is defined at `run-time` and is called a function expression, whereas function `bar` is defined at `parse time` and is called a function statement. To understand it better, let's take a look at the code below :
 
@@ -510,6 +541,8 @@ function bar() {
   console.log("Hi I am inside Foo");
 }
 ```
+</details>
+
 ## Question 17b. What is the output of the following?
 
 ```javascript
@@ -517,7 +550,7 @@ bar();
 (function abc(){console.log('something')})();
 function bar(){console.log('bar got called')};
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The output will be :
 ``` 
@@ -526,10 +559,11 @@ something
 ```
 Since the function is called first and defined during parse time the JS engine will try to find any possible parse time definitions and start the execution loop which will mean function is called first even if the definition is post another function.
 
-## Question 18. In which case the function definition is not hoisted in JavaScript?
+</details>
 
+## Question 18. In which case the function definition is not hoisted in JavaScript?
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 Let's take the following **function expression**
 
@@ -566,6 +600,8 @@ foo = function foo() {
 foo(); // Now foo is defined here
 ```
 
+</details>
+
 ## Question 19. What will be the output of the following code?
 
 ```javascript
@@ -579,7 +615,7 @@ var salary = "1000$";
   console.log("My New Salary " + salary);
 })();
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The code above will output: `undefined, 5000$` because of hoisting. In the code presented above, you might be expecting `salary` to retain it values from outer scope until the point that `salary` was re-declared in the inner scope. But due to `hoisting` salary value was `undefined` instead. To understand it better have a look of the following code, here `salary` variable is hoisted and declared at the top in function scope. When we print its value using `console.log` the result is `undefined`. Afterwards the variable is redeclared and the new value `"5000$"` is assigned to it.
 
@@ -596,9 +632,11 @@ var salary = "1000$";
 })();
 ```
 
+</details>
+
 ## Question 20. What’s the difference between `typeof` and `instanceof`?
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 `typeof` is an operator that returns a string with the type of whatever you pass.
 
@@ -623,6 +661,8 @@ name instanceof String; // Output : true
 
 Ref Link: [http://stackoverflow.com/questions/2449254/what-is-the-instanceof-operator-in-javascript](http://stackoverflow.com/questions/2449254/what-is-the-instanceof-operator-in-javascript)
 
+</details>
+
 ## Question 21. Calculate the length of the associative array
 
 ```javascript
@@ -632,7 +672,7 @@ var counterArray = {
 };
 counterArray["C"] = 1;
 ```
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 First of all, in case of JavaScript an associative array is the same as an object. Secondly, even though is no built-in function or property available to calculate the length/size an object, we can write such function ourselves.
 
@@ -676,7 +716,12 @@ _.size({one: 1, two: 2, three: 3});
 => 3
 ```
 
+</details>
+
 ## Question 22. Difference between `Function`, `Method` and `Constructor` calls in JavaScript.
+
+<details><summary><b>Answer</b></summary>
+
 If your are familiar with Object-oriented programming, More likely familiar to thinking of functions, methods, and class constructors as three separate things. But In JavaScript, these are just three different usage patterns of one single construct.
 
 functions : The simplest usages of function call:
@@ -731,6 +776,8 @@ Unlike function calls and method calls, a constructor call `new Employee('John D
 The primary role of the constructor function is to initialize the object.
 
 
+</details>
+
 ## Question 23. What would be the output of the following code?
 
 ```javascript
@@ -742,7 +789,7 @@ var person = new User("xyz")["location"] = "USA";
 console.log(person);
 ```
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The output of above code would be `"USA"`. Here `new User("xyz")` creates a brand new object and created property `location` on that and `USA` has been assigned to object property location and that has been referenced by the person.
 
@@ -763,9 +810,11 @@ foo["location"] = "USA";
 ```
  
  
+</details>
+
 ## Question 24. What are Service Workers and when can you use them?
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 It’s a technology that allows your web application to use cached resources first, and provide default experience offline, before getting more data from the network later. This principle is commonly known as Offline First.
 
@@ -773,7 +822,12 @@ Service Workers actively use promises. A Service Worker has to be installed,acti
 
 As of 2017, Service Workers are not supported in IE and Safari.
 
+</details>
+
 ## Question 25. What is the difference between a method and a function in javascript?
+
+<details><summary><b>Answer</b></summary>
+
 In JS, that difference is quite subtle. A function is a piece of code that is called by name and function itself not associated with any object and not defined inside any object. It can be passed data to operate on (i.e. parameter) and can optionally return data (the return value).
 
 ```javascript
@@ -857,8 +911,11 @@ obj1.myMethod(); // will print "Hi there" following with obj1.
 
 ```
 
+</details>
+
 ## Question 26. What is IIFE (Immediately Invoked Function Expression) and how it can be useful?
-### Answer 
+<details><summary><b>Answer</b></summary>
+
 #### Definition
 IIFE a function that runs as soon as it's defined. Usually it's anonymous (doesn't have a function name), but it also can be named. Here's an example of IIFE:
 
@@ -923,8 +980,10 @@ Variables and functions that you declare inside an IIFE are not visible to the o
 code, it helps to prevent polluting the global scope and provide the module interface to the outside.
 
 
+</details>
+
 ## Question 27. Describe Singleton Pattern In JavaScript
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 The singleton pattern is an often used JavaScript design pattern. It provides a way to wrap the code into a logical unit that can be accessed through a single variable. The Singleton design pattern is used when only one instance of an object is needed throughout the lifetime of an application. In JavaScript, Singleton pattern have many uses, they can be used for NameSpacing, which reduce the number of global variables in your page (prevent from polluting global space), organizing the code in a consistent manner, which increase the readability and maintainability of your pages.
 
@@ -1027,9 +1086,11 @@ console.log(MyNamespace.Singleton.getInstance().publicMethod());
 
 The singleton implemented above is easy to understand. The singleton class maintains a static reference to the lone singleton instance and return that reference from the static getInstance() method.
 
+</details>
+
 ## Question 28. What are the ways of creating objects in JavaScript ?
 
-### Answer 
+<details><summary><b>Answer</b></summary>
 
 #### Method 1: Function based
 
@@ -1095,12 +1156,14 @@ employee.getName = function(){
 
 `Object.create(obj)` will create a new object and set the `obj` as its prototype. It’s a modern way to create objects that inherit properties from other objects. `Object.create` function doesn’t run the constructor. You can use `Object.create(null)` when you don’t want your object to inherit the properties of `Object`.
 
+</details>
+
 ## Question 29. Write a function called deepClone which takes an object and creates a object copy of it.
 
 ``` javascript
 var newObject = deepClone(obj);
 ```
-Solution:
+<details><summary><b>Answer</b></summary>
 
 ```javascript
 function deepClone(object){
@@ -1133,8 +1196,12 @@ var personalDetail = {
 ```
 So when we do deep clone then we should copy every property (including the nested object).
 
+</details>
+
 ## Question 30. Best way to detect `undefined` object property in JavaScript.
 
+<details><summary><b>Answer</b></summary>
+
 > Suppose we have given an object `person`
 
 ```javascript
@@ -1161,6 +1228,8 @@ if(typeof person.salary === 'undefined'){
 	console.log("salary is undefined here because we haven't declared");
 }
 ```
+</details>
+
 ## Question 31. Write a function called `Clone` which takes an object and creates a object copy of it but not copy deep property of object. 
 
 ```javascript
@@ -1169,7 +1238,7 @@ if(typeof person.salary === 'undefined'){
 	console.log(cloneObj === Clone(objectLit)); // this should return false
 	console.log(cloneObj == Clone(objectLit)); // this should return true
 ```
-**solution:** 
+<details><summary><b>Answer</b></summary>
 
 ```javascript
 function Clone(object){
@@ -1181,8 +1250,12 @@ function Clone(object){
 }
 ```
 
+</details>
+
 ## Question 32. What are promises and how they are useful?
 
+<details><summary><b>Answer</b></summary>
+
 We use promises for handling asynchronous interactions in a sequential manner. They are especially useful when we need to do an async operation and THEN do another async operation based on the results of the first one. For example, if you want to request the list of all flights and then for each flight you want to request some details about it. The promise represents the future value. It has an internal state (`pending`, `fulfilled` and `rejected`) and works like a state machine.
 
 A promise object has `then` method, where you can specify what to do when the promise is fulfilled or rejected.
@@ -1196,8 +1269,12 @@ Also mention that you know about more sophisticated concepts:
 Be sure that you can implement the promise, read [one of the articles on a topic](https://opensourceconnections.com/blog/2014/02/16/a-simple-promise-implementation-in-about-20-lines-of-javascript/), and learn the source code of the [simplest promise implementation](https://gist.github.com/softwaredoug/9044640). 
 
 
+</details>
+
 ## Question 33. How to check whether a key exist in a JavaScript object or not.
 
+<details><summary><b>Answer</b></summary>
+
 Let say we have `person` object with property **name** and **age**
 
 ```javascript
@@ -1233,7 +1310,12 @@ console.log(person.hasOwnProperty('name')); // print true
 console.log(person.hasOwnProperty('salary')); // print false
 ```
 
+</details>
+
 ## Question 34. What is NaN, why do we need it, and when can it break the page?
+
+<details><summary><b>Answer</b></summary>
+
 `NaN` stands for “not a number.” and it can break your table of numbers when it has an arithmetic operation that is not allowed. Here are some examples of how you can get `NaN`:
 
 ```javascript
@@ -1255,6 +1337,8 @@ To check if the current value of the variable is NaN, you have to use the `isNaN
 
 Further reading: [great blogpost on ariya.io](https://ariya.io/2014/05/the-curious-case-of-javascript-nan)
 
+</details>
+
 ## Question 35. Fix the bug using ES5 only
 
 ```javascript
@@ -1265,6 +1349,8 @@ for (var i = 0; i < arr.length; i++) {
   }, 3000);
 }
 ```
+<details><summary><b>Answer</b></summary>
+
 For ES6, you can just replace `var i` with `let i`. 
 
 For ES5, you need to create a function scope like here:
@@ -1291,8 +1377,12 @@ arr.forEach(function(ele, i) {
 })
 ```
 
+</details>
+
 ## Question 36. How to check if the value of a variable in an array?
 
+<details><summary><b>Answer</b></summary>
+
 We always encounter in such situation where we need to know whether value is type of array or not.
 
 For instance : the code below perform some operation based value type
@@ -1382,8 +1472,12 @@ function isArray(value) {
 }
 ```
 
+</details>
+
 ## Question 37. Best way to detect reference values of any type in JavaScript ?
 
+<details><summary><b>Answer</b></summary>
+
  In Javascript Object are called as reference type, Any value other then primitive is definitely a reference type. There are several built-in reference type such as **Object**, **Array**, **Function**, **Date**, **null** and **Error**.
 
 Detecting object using `typeof` operator
@@ -1424,8 +1518,12 @@ console.log(emp1 instanceof Employee); // true
 console.log(emp1 instanceof Object); // true
 ```
 
+</details>
+
 ## Question 38. How does Object.create method works JavaScript?
 
+<details><summary><b>Answer</b></summary>
+
 The ECMAScript 5 **Object.create()** method is the easiest way for one object to inherit from another, without invoking a constructor function. 
 
 **For instance:** 
@@ -1473,8 +1571,12 @@ In the example above, `emp1` is created with it's own value for name, so calling
 
 Object created in this manner give you full control over newly created object. You are free to add, remove any properties and method you want.
 
+</details>
+
 ## Question 39. How to use constructor functions for inheritance in JavaScript?
 
+<details><summary><b>Answer</b></summary>
+
 Let say we have `Person` class which has name, age, salary properties and **incrementSalary()** method.
 
 ```javascript
@@ -1532,8 +1634,12 @@ console.log(name in obj); // true
 ```
 Type-based inheritance is best used with developer defined constructor function rather than natively in JavaScript. In addition to this also allows flexibility in how we create similar type of object.
 
+</details>
+
 ## Question 40. How we can prevent modification of object in JavaScript ?.
 
+<details><summary><b>Answer</b></summary>
+
  ECMAScript 5 introduce several methods to prevent modification of object which lock down object to ensure that no one, accidentally or otherwise, change functionality of Object.
 
 There are three levels of preventing modification: 
@@ -1642,9 +1748,13 @@ delete employee.name;  // fails silently unless it's in strict mode
 ``` 
 
 
-## Question 44. Write a log function which will add prefix `(your message)` to every message you log using console.log ? 
+</details>
+
+## Question 41. Write a log function which will add prefix `(your message)` to every message you log using console.log ? 
  For example, If you log `console.log("Some message")` then output should be **(your message) Some message**
 
+ <details><summary><b>Answer</b></summary>
+
 Logging error message or some informative message is always required when you dealing with client side JavaScript using console.log method. Some time you want to add some prefix to identify message generated log from your application hence you would like to prefix your app name in every console.log. 
 
 A general way to do this keep adding your app name in every console.log message like 
@@ -1667,7 +1777,9 @@ appLog("Some error message");
 //output of above console: 'your app name Some error message'
 ```
 
-## Question 45 . Write a function which will test string as a literal and as an object ?
+</details>
+
+## Question 42 . Write a function which will test string as a literal and as an object ?
 
 For example: We can create string using string literal and using String constructor function. 
 
@@ -1677,6 +1789,9 @@ For example: We can create string using string literal and using String construc
  // using String constructor function 
  var objStr = new String("Hi I am string object");
 ```
+
+<details><summary><b>Answer</b></summary>
+
  We can use typeof operator to test string literal and instanceof operator to test String object.
 
 ```javascript
@@ -1689,7 +1804,11 @@ For example: We can create string using string literal and using String construc
  console.log(isString(ltrlStr)); // true
  console.log(isString(objStr)); // true
 ``` 
-## Question 46 . What is typical use case for anonymous function in JavaScript ?
+</details>
+
+## Question 43 . What is typical use case for anonymous function in JavaScript ?
+
+<details><summary><b>Answer</b></summary>
 
  Anonymous functions basically used in following scenario.
 
@@ -1752,7 +1871,11 @@ If your answer is yes then go and create named function rather anonymous functio
 1. It can reduce a bit of code, particularly in recursive function and in callback function.
 2.  Avoid needless global namespace pollutions.
 
-## Question 47 . How to set a default parameter value ?
+</details>
+
+## Question 44 . How to set a default parameter value ?
+
+<details><summary><b>Answer</b></summary>
 
  If you are coming from python/c# you might be using default value for function parameter incase value(formal parameter) has not been passed. For instance : 
 
@@ -1832,7 +1955,9 @@ sentEmail({
 }, 'Yahoo Mail');
 ```
 
-## Question 48. Write code for merge two JavaScript Object dynamically.
+</details>
+
+## Question 45. Write code for merge two JavaScript Object dynamically.
 Let say you have two objects 
 
 ```javascript
@@ -1849,6 +1974,8 @@ var address = {
 ```
 Write merge function which will take two object and add all the own property of second object into first object.
 
+<details><summary><b>Answer</b></summary>
+
 ```javascript
 merge(person , address); 
  
@@ -1879,7 +2006,11 @@ function merge(toObj, fromObj) {
   }
 }
 ```
-## Question 49. What is non-enumerable property in JavaScript and how you can create one?
+</details>
+
+## Question 46. What is non-enumerable property in JavaScript and how you can create one?
+
+<details><summary><b>Answer</b></summary>
 
 Object can have properties that don't show up when you iterate through object using for...in loop or using Object.keys() to get an array of property names. This properties is know as non-enumerable properties.
 
@@ -1925,7 +2056,11 @@ person.phoneNo = '7777777777';
 
 **Object.defineProperty()** also lets you create read-only properties as we saw above, we are not able to modify phoneNo value of a person object. This is because descriptor has **writable** property, which is `false` by default. Changing non-writable property value will return error in strict mode. In non-strict mode it won't through any error but it won't change the value of phoneNo.
 
-## Question 50. What is Function binding ?
+</details>
+
+## Question 47. What is Function binding ?
+
+<details><summary><b>Answer</b></summary>
 
  Function binding falls in advance JavaScript category and this is very popular technique to use in conjunction with event handler and callback function to preserve code execution context while passing function as a parameter.
 
@@ -1965,6 +2100,7 @@ btn.addEventListener('click', clickHandler.handleClick.bind(clickHandler));
 
 `bind` method is available to all the function similar to call and apply method which take argument value of `this`.
 
+</details>
 
 # Coding Questions
 
@@ -1982,20 +2118,29 @@ strB="bye there!";
 console.log (strA)
 ```
 
-The output will `'hi there'` because we're dealing with strings here. Strings are 
+<details><summary><b>Answer</b></summary>
+
+The output will be `'hi there'` because we're dealing with strings here. Strings are 
 passed by value, that is, copied. 
- 
-### 2. What would be the output of following code?
+
+</details>
+
+###  2. What would be the output of following code?
 ```javascript
 var objA = {prop1: 42};
 var objB = objA; 
 objB.prop1 = 90;
 console.log(objA) 
 ```
-The output will `{prop1: 90}` because we're dealing with objects here. Objects are 
+
+<details><summary><b>Answer</b></summary>
+
+The output will be `{prop1: 90}` because we're dealing with objects here. Objects are 
 passed by reference, that is, `objA` and `objB` point to the same object in memory. 
 
-### 3. What would be the output of following code?
+</details>
+
+###  3. What would be the output of following code?
 
 ```javascript
 var objA = {prop1: 42};
@@ -2004,7 +2149,10 @@ objB = {};
 console.log(objA)
 ```
 
-The output will `{prop1: 42}`. 
+
+<details><summary><b>Answer</b></summary>
+
+The output will be `{prop1: 42}`. 
 
 When we assign `objA` to `objB`, the `objB` variable will point
 to the same object as the `objB` variable.
@@ -2012,7 +2160,9 @@ to the same object as the `objB` variable.
 However, when we reassign `objB` to an empty object, we simply change where `objB` variable references to.
 This doesn't affect where `objA` variable references to. 
 
-### 4. What would be the output of following code?
+</details>
+
+###  4. What would be the output of following code?
 
 ```javascript
 var arrA = [0,1,2,3,4,5];
@@ -2021,13 +2171,18 @@ arrB[0]=42;
 console.log(arrA)
 ```
 
+
+<details><summary><b>Answer</b></summary>
+
 The output will be `[42,1,2,3,4,5]`. 
 
 Arrays are object in JavaScript and they are passed and assigned by reference. This is why
 both `arrA` and `arrB` point to the same array `[0,1,2,3,4,5]`. That's why changing the first
 element of the `arrB` will also modify `arrA`: it's the same array in the memory.
 
-### 5. What would be the output of following code?
+</details>
+
+###  5. What would be the output of following code?
 ```javascript
 var arrA = [0,1,2,3,4,5];
 var arrB = arrA.slice();
@@ -2035,12 +2190,17 @@ arrB[0]=42;
 console.log(arrA)
 ```
 
+
+<details><summary><b>Answer</b></summary>
+
 The output will be `[0,1,2,3,4,5]`. 
 
 The `slice` function copies all the elements of the array returning the new array. That's why
 `arrA` and `arrB` reference two completely different arrays. 
 
-### 5. What would be the output of following code?
+</details>
+
+###  6. What would be the output of following code?
 
 ```javascript
 var arrA = [{prop1: "value of array A!!"},  {someProp: "also value of array A!"}, 3,4,5];
@@ -2048,13 +2208,17 @@ var arrB = arrA;
 arrB[0].prop1=42;
 console.log(arrA);
 ```
+
+<details><summary><b>Answer</b></summary>
+
 The output will be `[{prop1: 42},  {someProp: "also value of array A!"}, 3,4,5]`. 
 
 Arrays are object in JS, so both varaibles arrA and arrB point to the same array. Changing
 `arrB[0]` is the same as changing `arrA[0]`
 
+</details>
 
-### 6. What would be the output of following code?
+### 7. What would be the output of following code?
 
 ```javascript
 var arrA = [{prop1: "value of array A!!"}, {someProp: "also value of array A!"},3,4,5];
@@ -2064,6 +2228,8 @@ arrB[3] = 20;
 console.log(arrA);
 ```
 
+<details><summary><b>Answer</b></summary>
+
 The output will be `[{prop1: 42},  {someProp: "also value of array A!"}, 3,4,5]`. 
 
 The `slice` function copies all the elements of the array returning the new array. However,
@@ -2088,6 +2254,7 @@ two elements.
 
 This is why changing the property of `arrB[0]` in `arrB` will also change the `arrA[0]`.
 
+</details>
 
 ## Hoisting 
 
@@ -2098,9 +2265,13 @@ This is why changing the property of `arrB[0]` in `arrB` will also change the `a
 3.  Type Error
 4.  ReferenceError: employeeId is not defined 
 
-Answer: 4) ReferenceError: employeeId is not defined 
+<details><summary><b>Answer</b></summary>
 
-### 2. What would be the output of following code?
+ 4) ReferenceError: employeeId is not defined 
+
+</details>
+
+###  2. What would be the output of following code?
 
 ```javascript
 console.log(employeeId);
@@ -2112,7 +2283,11 @@ var employeeId = '19000';
 3.  Type Error
 4.  ReferenceError: employeeId is not defined 
 
-Answer: 2) undefined 
+<details><summary><b>Answer</b></summary>
+
+ 2) undefined 
+
+</details>
 
 ### 3. What would be the output of following code?
 
@@ -2129,7 +2304,11 @@ var employeeId = '1234abe';
 3.  Type Error
 4.  ReferenceError: employeeId is not defined 
 
-Answer: 2) undefined 
+<details><summary><b>Answer</b></summary>
+
+ 2) undefined 
+
+</details>
 
 ### 4. What would be the output of following code?
 
@@ -2149,7 +2328,11 @@ var employeeId = '1234abe';
 3.  '1234abe'
 4.  ReferenceError: employeeId is not defined 
 
-Answer: 2) undefined 
+<details><summary><b>Answer</b></summary>
+
+ 2) undefined 
+
+</details>
 
 ### 5. What would be the output of following code?
 
@@ -2167,7 +2350,11 @@ Answer: 2) undefined
 3.  'Hi I am inside displayFunc'
 4.  ReferenceError: displayFunc is not defined 
 
-Answer: 1) undefined 
+<details><summary><b>Answer</b></summary>
+
+ 1) undefined 
+
+</details>
 
 ### 6. What would be the output of following code?
 
@@ -2186,7 +2373,11 @@ console.log(employeeId);
 3.  'abc123'
 4.  ReferenceError: employeeId is not defined 
 
-Answer: 2) '123bcd' 
+<details><summary><b>Answer</b></summary>
+
+ 2) '123bcd' 
+
+</details>
 
 ### 7. What would be the output of following code?
 
@@ -2208,7 +2399,11 @@ console.log(employeeId);
 3.  'abc123'
 4.  ReferenceError: employeeId is not defined 
 
-Answer: 3) 'abc123' 
+<details><summary><b>Answer</b></summary>
+
+ 3) 'abc123' 
+
+</details>
 
 ### 8. What would be the output of following code?
 
@@ -2231,7 +2426,11 @@ foo();
 3.  string
 4.  ReferenceError: employeeId is not defined 
 
-Answer: 2) 'function'
+<details><summary><b>Answer</b></summary>
+
+ 2) 'function'
+
+</details>
 
 ### 9. What would be the output of following code?
 
@@ -2253,7 +2452,11 @@ foo();
 3.  'Car'
 4.  ReferenceError: product is not defined 
 
-Answer: 1) undefined
+<details><summary><b>Answer</b></summary>
+
+ 1) undefined
+
+</details>
 
 ### 10. What would be the output of following code?
 
@@ -2277,7 +2480,11 @@ Answer: 1) undefined
 3.  function function
 4.  ReferenceError: bar is not defined 
 
-Answer: 3) function function
+<details><summary><b>Answer</b></summary>
+
+ 3) function function
+
+</details>
 
 ## Objects
 
@@ -2306,7 +2513,11 @@ Answer: 3) function function
 3.  ["name", "salary", "country", "phoneNo"]
 4.  ["name", "salary", "country"]
 	
-Answer: 3) ["name", "salary", "country", "phoneNo"]
+<details><summary><b>Answer</b></summary>
+
+ 3) ["name", "salary", "country", "phoneNo"]
+
+</details>
 
 ### 2. What would be the output of following code ?
 
@@ -2333,7 +2544,11 @@ Answer: 3) ["name", "salary", "country", "phoneNo"]
 3.  ["name", "salary", "country", "phoneNo"]
 4.  ["name", "salary", "country"]
 	
-Answer: 4) ["name", "salary", "country"]
+<details><summary><b>Answer</b></summary>
+
+ 4) ["name", "salary", "country"]
+
+</details>
 
 ### 3. What would be the output of following code ?
 
@@ -2356,7 +2571,11 @@ Answer: 4) ["name", "salary", "country"]
 3.  true false
 4.  true true
 	
-Answer: 2) false false
+<details><summary><b>Answer</b></summary>
+
+ 2) false false
+
+</details>
 
 ### 4. What would be the output of following code ?
 
@@ -2373,7 +2592,11 @@ Answer: 2) false false
 3.  true false
 4.  true true
 	
-Answer: 2) false false
+<details><summary><b>Answer</b></summary>
+
+ 2) false false
+
+</details>
 
 ### 5. What would be the output of following code ?
 
@@ -2394,7 +2617,11 @@ Answer: 2) false false
 3.  true false
 4.  true true
 	
-Answer: 2) false false
+<details><summary><b>Answer</b></summary>
+
+ 2) false false
+
+</details>
 
 ### 6. What would be the output of following code ?
 
@@ -2413,7 +2640,11 @@ Answer: 2) false false
 3.  true false
 4.  true true
 	
-Answer: 2) false false
+<details><summary><b>Answer</b></summary>
+
+ 2) false false
+
+</details>
 
 ### 7. What would be the output of following code ?
 
@@ -2432,7 +2663,11 @@ Answer: 2) false false
 3.  true false
 4.  true true
 	
-Answer: 4) true true
+<details><summary><b>Answer</b></summary>
+
+ 4) true true
+
+</details>
 
 ### 8. What would be the output of following code ?
 
@@ -2453,7 +2688,11 @@ Answer: 4) true true
 3.  true true true true
 4.  true true false false
 	
-Answer: 3) true true true true
+<details><summary><b>Answer</b></summary>
+
+ 3) true true true true
+
+</details>
 
 ### 9. What would be the output of following code ?
 
@@ -2473,7 +2712,11 @@ Answer: 3) true true true true
 3.  foo foo
 4.  bar foo
 	
-Answer: 2) bar bar
+<details><summary><b>Answer</b></summary>
+
+ 2) bar bar
+
+</details>
 
 ### 10. What would be the output of following code ?
 
@@ -2495,7 +2738,11 @@ Answer: 2) bar bar
 3.  foo foo
 4.  bar foo
 	
-Answer: 3) foo foo
+<details><summary><b>Answer</b></summary>
+
+ 3) foo foo
+
+</details>
 
 ### 11. What would be the output of following code ?
 
@@ -2517,7 +2764,11 @@ Answer: 3) foo foo
 3.  foo foo
 4.  undefined bar
 	
-Answer: 2) undefined undefined 
+<details><summary><b>Answer</b></summary>
+
+ 2) undefined undefined
+
+</details>
 
 ## Arrays
 
@@ -2536,7 +2787,11 @@ Answer: 2) undefined undefined
 3.  ["100"] 1
 4.  ReferenceError: array is not defined 
 
-Answer: 3) ["100"] 1
+<details><summary><b>Answer</b></summary>
+
+ 3) ["100"] 1
+
+</details>
 
 ### 2. What would be the output of following code?
 
@@ -2557,7 +2812,11 @@ Answer: 3) ["100"] 1
 3.  [] [] ['1',2,'3',4,5.6] 5
 4.  [] [] [Array[5]] 5 
 
-Answer: 1) [] [] [Array[5]] 1
+<details><summary><b>Answer</b></summary>
+
+ 1) [] [] [Array[5]] 1
+
+</details>
 
 ### 3. What would be the output of following code?
 
@@ -2575,7 +2834,11 @@ Answer: 1) [] [] [Array[5]] 1
 3.  6
 4.  undefined
 
-Answer: 1) 11
+<details><summary><b>Answer</b></summary>
+
+ 1) 11
+
+</details>
 
 ### 4. What would be the output of following code?
 
@@ -2593,7 +2856,11 @@ Answer: 1) 11
 3.  6
 4.  undefined
 
-Answer: 3) 6
+<details><summary><b>Answer</b></summary>
+
+ 3) 6
+
+</details>
 
 ### 5. What would be the output of following code?
 
@@ -2611,7 +2878,11 @@ Answer: 3) 6
 3.  Type Error
 4.  undefined
 
-Answer: 1) [ 'dog', 'rat', 'goat', 'cow', 'horse', 'cat' ]
+<details><summary><b>Answer</b></summary>
+
+ 1) [ 'dog', 'rat', 'goat', 'cow', 'horse', 'cat' ]
+
+</details>
 
 ### 6. What would be the output of following code?
 
@@ -2630,7 +2901,11 @@ Answer: 1) [ 'dog', 'rat', 'goat', 'cow', 'horse', 'cat' ]
 3.  1 -1 -1 -1
 4.  1 undefined -1 4
 
-Answer: 1) 1 -1 -1 4
+<details><summary><b>Answer</b></summary>
+
+ 1) 1 -1 -1 4
+
+</details>
 
 ### 7. What would be the output of following code?
 
@@ -2648,7 +2923,11 @@ Answer: 1) 1 -1 -1 4
 3.  1 1 -1 
 4.  1 undefined undefined
 
-Answer: 2) 1 6 -1
+<details><summary><b>Answer</b></summary>
+
+ 2) 1 6 -1
+
+</details>
 
 ### 8. What would be the output of following code?
 
@@ -2673,7 +2952,11 @@ Answer: 2) 1 6 -1
 3.  [ 2, 4, 8, 12, 16 ] true 
 4.  [ 2, 4, 8, 12, 16 ] false
 
-Answer: 3) [ 2, 4, 8, 12, 16 ] true 
+<details><summary><b>Answer</b></summary>
+
+ 3) [ 2, 4, 8, 12, 16 ] true 
+
+</details>
 
 ### 9. What would be the output of following code?
 
@@ -2709,11 +2992,15 @@ Answer: 3) [ 2, 4, 8, 12, 16 ] true
 	[ 2, '12', true,false]
 
 
-Answer: 1) [ 2, '12', true ]
+<details><summary><b>Answer</b></summary>
+
+ 1) [ 2, '12', true ]
 			 [ 2, '12', true ]
 			 [ 2, '12', true ]
 			 [ 2, '12', true ]
 			 
+</details>
+
 ### 10. What would be the output of following code?
 
 ```javascript
@@ -2748,12 +3035,16 @@ Answer: 1) [ 2, '12', true ]
    []
    [ 'foo', 'bar', 'john', 'ritz' ]
 
-Answer: 1) [ 'bar', 'john', 'ritz' ]
+<details><summary><b>Answer</b></summary>
+
+ 1) [ 'bar', 'john', 'ritz' ]
 		 	 [ 'bar', 'john' ]
            [ 'foo', 'bar', 'john', 'ritz' ]
            []
            [ 'foo', 'bar', 'john', 'ritz' ]		
 
+</details>
+
 ### 11. What would be the output of following code?
 
 ```javascript
@@ -2770,7 +3061,11 @@ Answer: 1) [ 'bar', 'john', 'ritz' ]
 3.  [ 'bar', 'john' ] [ 'bar', 'john' ] [ 'bar', 'john' ]
 4.  [ 'bar', 'john' ] [] []
 
-Answer: 1.  [ 'bar', 'john' ] [] [ 'foo' ] 
+<details><summary><b>Answer</b></summary>
+
+ 1.  [ 'bar', 'john' ] [] [ 'foo' ] 
+
+</details>
 
 ### 12. What would be the output of following code?
 
@@ -2787,7 +3082,11 @@ Answer: 1.  [ 'bar', 'john' ] [] [ 'foo' ]
 3.  [ 15, 16, 2, 23, 42, 8 ]
 4.  [ 2, 8, 15, 16, 23, 42 ]
 
-Answer: 3.  [ 15, 16, 2, 23, 42, 8 ]
+<details><summary><b>Answer</b></summary>
+
+ 3.  [ 15, 16, 2, 23, 42, 8 ]
+
+</details>
 
 ## Functions
 
@@ -2814,7 +3113,11 @@ console.log(funcA());
 3.  Type Error
 4.  ReferenceError: this is not defined 
 	
-Answer: 1) 
+<details><summary><b>Answer</b></summary>
+
+ 1) 
+
+</details>
 
 ### 2. What would be the output of following code ?
 
@@ -2834,7 +3137,11 @@ console.log(obj.innerMessage);
 3.  Type Error
 4.  undefined true
 	
-Answer: 4) undefined true
+<details><summary><b>Answer</b></summary>
+
+ 4) undefined true
+
+</details>
 
 ### 3. What would be the output of following code ?
 
@@ -2854,7 +3161,11 @@ console.log(obj.innerMessage());
 3.  Type Error
 4.  ReferenceError: this.message is not defined
 	
-Answer: 1) Hello
+<details><summary><b>Answer</b></summary>
+
+ 1) Hello
+
+</details>
 
 ### 4. What would be the output of following code ?
 
@@ -2875,8 +3186,12 @@ console.log(obj.innerMessage());
 3.  undefined
 4.  ReferenceError: this.message is not defined
 	
-Answer: 3) undefined
+<details><summary><b>Answer</b></summary>
+
+ 3) undefined
 	
+</details>
+
 ### 5. What would be the output of following code ?
 
 ```javascript
@@ -2897,7 +3212,11 @@ console.log(obj.innerMessage());
 3.  undefined
 4.  ReferenceError: self.message is not defined
 	
-Answer: 2) 'Hello'
+<details><summary><b>Answer</b></summary>
+
+ 2) 'Hello'
+
+</details>
 
 ### 6. What would be the output of following code ?
 
@@ -2915,7 +3234,11 @@ console.log(myFunc());
 3.  undefined
 4.  ReferenceError: this.message is not defined
 	
-Answer: 3) undefined
+<details><summary><b>Answer</b></summary>
+
+ 3) undefined
+
+</details>
 
 ### 7. What would be the output of following code ?
 
@@ -2933,7 +3256,11 @@ console.log(myFunc());
 3.  undefined
 4.  ReferenceError: this.message is not defined
 	
-Answer: 2) 'Hi John'
+<details><summary><b>Answer</b></summary>
+
+ 2) 'Hi John'
+
+</details>
 
 ### 8. What would be the output of following code ?
 
@@ -2950,7 +3277,11 @@ console.log(myFunc());
 3.  undefined
 4.  ReferenceError: this.message is not defined
 	
-Answer: 2) 'Hi John'
+<details><summary><b>Answer</b></summary>
+
+ 2) 'Hi John'
+
+</details>
 
 ### 9. What would be the output of following code ?
 
@@ -2968,7 +3299,11 @@ console.log(myFunc("a","b","c","d"));
 3.  undefined
 4.  ReferenceError
 	
-Answer: a) 2 2 2 
+<details><summary><b>Answer</b></summary>
+
+ a) 2 2 2 
+
+</details>
 
 ### 10. What would be the output of following code ?
 
@@ -2986,7 +3321,11 @@ console.log(myFunc("a","b","c","d"));
 3.  undefined
 4.  ReferenceError
 	
-Answer: 2) 0 2 4 
+<details><summary><b>Answer</b></summary>
+
+ 2) 0 2 4 
+
+</details>
 
 ## Object Oriented
 
@@ -3016,7 +3355,11 @@ var person1 = new Person('John');
 3.  John undefined
 4.  John John
 	
-Answer: 1) John Person 
+<details><summary><b>Answer</b></summary>
+
+ 1) John Person 
+
+</details>
 
 ## Scopes
 
@@ -3041,7 +3384,11 @@ console.log(userInfo.userName);
 3.  12345678 undefined
 4.  undefined undefined
 	
-Answer: 3) 12345678 undefined 
+<details><summary><b>Answer</b></summary>
+
+ 3) 12345678 undefined 
+
+</details>
 
 ### 2. What would be the output of following code ?
 
@@ -3058,7 +3405,11 @@ console.log(Employee.employeeId);
 3.  bq1uy
 4.  undefined
 	
-Answer: 4) undefined 
+<details><summary><b>Answer</b></summary>
+
+ 4) undefined 
+
+</details>
 
 ### 3. What would be the output of following code ?
 
@@ -3080,7 +3431,11 @@ console.log(new Employee().employeeId);
 3.  bq1uy 1BJKSJ kj182
 4.  undefined 1BJKSJ kj182
 	
-Answer: 2) bq1uy 1BJKSJ bq1uy 
+<details><summary><b>Answer</b></summary>
+
+ 2) bq1uy 1BJKSJ bq1uy 
+
+</details>
 
 ### 4. What would be the output of following code ?
 
@@ -3101,7 +3456,11 @@ var employeeId = 'aq123';
 3.  aq123 aq123
 4.  foo123 undefined 
 	
-Answer: 1) foo123 aq123 
+<details><summary><b>Answer</b></summary>
+
+ 1) foo123 aq123 
+
+</details>
 
 ## Call, Apply, Bind
 
@@ -3122,7 +3481,11 @@ Answer: 1) foo123 aq123
 3.  World
 4.  [ 'W', 'o', 'r', 'l', 'd' ] 
 	
-Answer: 4) [ 'W', 'o', 'r', 'l', 'd' ]  
+<details><summary><b>Answer</b></summary>
+
+ 4) [ 'W', 'o', 'r', 'l', 'd' ]  
+
+</details>
 
 ### 2. What would be the output of following code ?
 
@@ -3153,7 +3516,11 @@ Answer: 4) [ 'W', 'o', 'r', 'l', 'd' ]
 3.  Total amount left in account: 3600 Total amount left in account: 3300
 4.  Total amount left in account: 5600 Total amount left in account: 5600
 	
-Answer: 1) Total amount left in account: 5600 Total amount left in account: 5300 
+<details><summary><b>Answer</b></summary>
+
+ 1) Total amount left in account: 5600 Total amount left in account: 5300 
+
+</details>
 
 ### 3. What would be the output of following code ?
 
@@ -3185,7 +3552,11 @@ Answer: 1) Total amount left in account: 5600 Total amount left in account: 5300
 3. 5600 3300 5100
 4. undefined undefined undefined
 	
-Answer: 1) 5600 5300 5100
+<details><summary><b>Answer</b></summary>
+
+ 1) 5600 5300 5100
+
+</details>
 
 ### 4. What would be the output of following code ?
 
@@ -3217,7 +3588,11 @@ Answer: 1) 5600 5300 5100
 3. 5600 3300 5100
 4. undefined undefined undefined
 	
-Answer: 2) 3600 3300 3100
+<details><summary><b>Answer</b></summary>
+
+ 2) 3600 3300 3100
+
+</details>
 
 ### 5. What would be the output of following code ?
 
@@ -3234,7 +3609,11 @@ Answer: 2) 3600 3300 3100
 3. Window
 4. undefined
 	
-Answer: 1) Hello John
+<details><summary><b>Answer</b></summary>
+
+ 1) Hello John
+
+</details>
 
 ### 6. Suggest your question!
 
@@ -3264,7 +3643,11 @@ getDataFromServer('www.google.com').then(function(name){
 3. Reference Error
 4. fn is not defined
 	
-Answer: 1) John
+<details><summary><b>Answer</b></summary>
+
+ 1) John
+
+</details>
 
 ### 2. What would be the output of following code ?
 
@@ -3308,9 +3691,13 @@ Answer: 1) John
    [42, 23, 16, 15, 8, 2]
 4. Reference Error
 	
-Answer: 1) [ 2, 8, 15, 16, 23, 42 ]
+<details><summary><b>Answer</b></summary>
+
+ 1) [ 2, 8, 15, 16, 23, 42 ]
 			 [ 2, 8, 15, 16, 23, 42 ]
 			 [ 2, 8, 15, 16, 23, 42 ]
+
+</details>
 			
 ## Return Statement
 
@@ -3334,7 +3721,11 @@ Answer: 1) [ 2, 8, 15, 16, 23, 42 ]
 3. Reference Error
 4. Uncaught TypeError: Cannot read property 'fullName' of undefined
 	
-Answer: 4) Uncaught TypeError: Cannot read property 'fullName' of undefined
+<details><summary><b>Answer</b></summary>
+
+ 4) Uncaught TypeError: Cannot read property 'fullName' of undefined
+
+</details>
 
 ### 2. What would be the output of following code ?
 
@@ -3352,7 +3743,11 @@ console.log(numb);
 3. 2
 4. (2,4,5)
 	
-Answer: 1) 5
+<details><summary><b>Answer</b></summary>
+
+ 1) 5
+
+</details>
 
 ### 3. What would be the output of following code ?
 
@@ -3370,7 +3765,11 @@ console.log(numb);
 3. ""
 4. 0
 	
-Answer: 2) undefined
+<details><summary><b>Answer</b></summary>
+
+ 2) undefined
+
+</details>
 
 ### 4**. What would be the output of following code ?
 
@@ -3392,7 +3791,11 @@ console.log(mul(2)(3)[1](4));
 3. Reference Error
 4. 10, 6
 	
-Answer: 1) 6, 10
+<details><summary><b>Answer</b></summary>
+
+ 1) 6, 10
+
+</details>
 
 ### 5**. What would be the output of following code ?
 
@@ -3416,7 +3819,11 @@ console.log(mul(2)(3).sum(4));
 3. Reference Error
 4. 10, 6
 	
-Answer: 1) 6, 10
+<details><summary><b>Answer</b></summary>
+
+ 1) 6, 10
+
+</details>
 
 ### 6. What would be the output of following code ?
 
@@ -3440,7 +3847,11 @@ console.log(mul(2)(3)(4)(5)(6));
 3. Reference Error
 4. Type Error
 	
-Answer: 1) 720
+<details><summary><b>Answer</b></summary>
+
+ 1) 720
+
+</details>
 
 ### 7. What would be the output of following code ?
 
@@ -3467,7 +3878,9 @@ personObj.getName2();
 3. undefined Tony
 4. Tony Tony
 
-Answer: 2) Tony undefined
+<details><summary><b>Answer</b></summary>
+
+ 2) Tony undefined
 
 Explaination: **getName1()** function works fine because it's being called from ***personObj***, so it has access to *this.name* property. But when while calling **getnName2** which is defined under *Object.prototype* doesn't have any proprty named *this.name*. There should be *name* property under prototype. Following is the code:
 
@@ -3490,6 +3903,7 @@ Object.prototype.name="Steve";
 personObj.getName2();
 ```
 
+</details>
 
 ## Contributing
 

From 93751de0c0d41dcf3fc38c3e742419317cced0b9 Mon Sep 17 00:00:00 2001
From: AlexTaietti <taiettialex@yahoo.it>
Date: Sun, 16 Aug 2020 02:40:08 +0100
Subject: [PATCH 40/46] corrected some typos

---
 README.md | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index 5a4ba4d..3f3f3b4 100644
--- a/README.md
+++ b/README.md
@@ -24,7 +24,7 @@ var x; // declaring x
 console.log(x); // output: undefined
 ```
 
-`var x = 1` is both declaration and definition (also we can say we are doing initialisation), Here declaration and assignment of value happen inline for variable x, In JavaScript every variable declaration and function declaration brings to the top of its current scope in which it's declared then assignment happen in order this term is called `hoisting`.
+`var x = 1` is both declaration and definition (what we are doing is called "initialisation"), Here declaration and assignment of value happen inline for variable x, In JavaScript both variable declarations and function declarations go to the top of the scope in which they are declared, then assignment happens—this series of events is called `hoisting`.
 
 A variable can be declared but not defined. When we try to access it, It will result `undefined`.
 
@@ -644,7 +644,7 @@ The `typeof` operator checks if a value belongs to one of the seven basic types:
 
 `typeof(null)` will return `object`.
 
-`instanceof` is much more intelligent: it works on the level of prototypes. In particular, it tests to see if the right operand appears anywhere in the prototype chain of the left. `instanceof` doesn’t work with primitive types. It `instanceof` operator checks the current object and returns true if the object is of the specified type, for example:
+`instanceof` is much more intelligent: it works on the level of prototypes. In particular, it tests to see if the right operand appears anywhere in the prototype chain of the left. `instanceof` doesn’t work with primitive types. The `instanceof` operator checks the current object and returns true if the object is of the specified type, for example:
 
 ```javascript
 var dog = new Animal();
@@ -674,11 +674,11 @@ counterArray["C"] = 1;
 ```
 <details><summary><b>Answer</b></summary>
 
-First of all, in case of JavaScript an associative array is the same as an object. Secondly, even though is no built-in function or property available to calculate the length/size an object, we can write such function ourselves.
+First of all, in the case of JavaScript an associative array is the same as an object. Secondly, even though there is no built-in function or property available to calculate the length/size an object, we can write such function ourselves.
 
 #### Method 1
 
-`Object` has `keys` method which can we used to calculate the length of object.
+`Object` has `keys` method which can be used to calculate the length of object.
 
 ```javascript
 Object.keys(counterArray).length; // Output 3

From b32e23ec3b80f94764b0e80a486ab6e1f11fa649 Mon Sep 17 00:00:00 2001
From: AlexTaietti <taiettialex@yahoo.it>
Date: Sat, 22 Aug 2020 21:17:49 +0100
Subject: [PATCH 41/46] corrected typos and rephrased some sentences

---
 README.md | 77 +++++++++++++++++++++++++++++--------------------------
 1 file changed, 41 insertions(+), 36 deletions(-)

diff --git a/README.md b/README.md
index 3f3f3b4..b8b329b 100644
--- a/README.md
+++ b/README.md
@@ -2,12 +2,12 @@
 
 # 123-JavaScript-Interview-Questions
 
-It's a book about frontend interview question. We hope that it will help all javascript developers to prepare for a technical job interview. 
+This book's goal is to help javascript frontend developers prepare for technical job interviews through a collection of carefully compiled questions.
 
 ## Want to buy a book in paper form? Want some badass flashcards?
 
- - This Book will be soon completed and then it will be available to buy in a paper form. If you want me to sent an early copy of this book, please add your name and email address in google form here [Google Form](https://goo.gl/forms/c8ubV1tWBBdz6fJP2).
- - If you don't want to wait, you can buy [Yuri's JavaScript Flashcards](http://flashcardsjs.com), a set of frontend interview questions sorted by popularity among the interviewers printed on beautiful poker-size flashcards.
+ - This Book will be soon completed and then it will be available to buy in paper form. If you want me to send you an early copy of this book, please add your name and email address in this [Google Form](https://goo.gl/forms/c8ubV1tWBBdz6fJP2).
+ - If you don't want to wait, you can buy [Yuri's JavaScript Flashcards](http://flashcardsjs.com), a set of frontend interview questions sorted by popularity among interviewers printed on beautiful poker-size flashcards.
 
 ## Question 1. What's the difference between `undefined` and `not defined` in JavaScript
 
@@ -17,14 +17,14 @@ In JavaScript if you try to use a variable that doesn't exist and has not been d
 
 Before starting further discussion let's understand the difference between declaration and definition.
 
-`var x` is a declaration because you are not defining what value it holds yet, but you are declaring its existence and the need for memory allocation.
+`var x` is a declaration because we are not defining what value it holds yet, but we are declaring its existence and the need for memory allocation.
 
 ```javascript
 var x; // declaring x
 console.log(x); // output: undefined
 ```
 
-`var x = 1` is both declaration and definition (what we are doing is called "initialisation"), Here declaration and assignment of value happen inline for variable x, In JavaScript both variable declarations and function declarations go to the top of the scope in which they are declared, then assignment happens—this series of events is called `hoisting`.
+`var x = 1` is both declaration and definition, here declaration and assignment of value happen inline for variable x—what we are doing is called "initialisation". In JavaScript both variable declarations and function declarations go to the top of the scope in which they are declared, then assignment happens—this series of events is called "hoisting".
 
 A variable can be declared but not defined. When we try to access it, It will result `undefined`.
 
@@ -48,7 +48,7 @@ console.log(y);  // Output: ReferenceError: y is not defined
 
 
 ```javascript
-//  if( x <= 100 ) {...}
+if( x <= 100 ) {...}
 if( !(x > 100) ) {...}
 ```
 <details><summary><b>Answer</b></summary>
@@ -56,9 +56,9 @@ if( !(x > 100) ) {...}
 `NaN <= 100` is `false` and `NaN > 100` is also `false`, so if the
 value of `x` is `NaN`, the statements are not the same.
 
-The same holds true for any value of x that being converted to Number, returns NaN, e.g.: `undefined`, `[1,2,5]`, `{a:22}` , etc.
+The same holds true for any value of x that being converted to type Number, returns `NaN`, e.g.: `undefined`, `[1,2,5]`, `{a:22}` , etc.
 
-This is why you need to pay attention when you deal with numeric variables. `NaN` can’t be equal, less than or more than any other numeric value, so the only reliable way to check if the value is `NaN`, is to use `isNaN()` function.
+This is why you need to pay attention when you deal with numeric variables. `NaN` can’t be equal, less than or more than any other numeric value, so the only reliable way to check if the value is `NaN`, is to use the `isNaN()` function.
 
 </details>
 
@@ -66,7 +66,7 @@ This is why you need to pay attention when you deal with numeric variables. `NaN
 
 <details><summary><b>Answer</b></summary>
 
-One of the drawback of declaring methods directly in JavaScript objects is that they are very memory inefficient.  When you do that, a new copy of the method is created for each instance of an object. Let's see it on example:
+One of the drawbacks of declaring methods directly in JavaScript objects is that they are very memory inefficient.  When you do that, a new copy of the method is created for each instance of an object. Here's an example:
 
 ```javascript
 var Employee = function (name, company, salary) {
@@ -80,7 +80,7 @@ var Employee = function (name, company, salary) {
   };
 };
 
-// we can also create method in Employee's prototype:
+// Alternatively we can add the method to Employee's prototype:
 Employee.prototype.formatSalary2 = function() {
     return "$ " + this.salary;
 }
@@ -91,7 +91,7 @@ var emp2 = new Employee('Dinesh Gupta', 'Company 2', 1039999);
 var emp3 = new Employee('Erich Fromm', 'Company 3', 1299483);
 ```
 
-Here each instance variable `emp1`, `emp2`, `emp3` has own copy of `formatSalary` method. However the `formatSalary2` will only be added once to an object `Employee.prototype`.
+In this case each instance variable `emp1`, `emp2`, `emp3` has its own copy of the`formatSalary` method. However the `formatSalary2` will only be added once to `Employee.prototype`.
 
 </details>
 
@@ -99,39 +99,45 @@ Here each instance variable `emp1`, `emp2`, `emp3` has own copy of `formatSalary
 
 <details><summary><b>Answer</b></summary>
 
-A closure is a function defined inside another function (called parent function) and has access to the variable which is declared and defined in parent function scope.
+A closure is a function defined inside another function (called parent function) and as such it has access to the variables declared and defined within its parent function's scope.
+
+The closure has access to the variables in three scopes:
 
-The closure has access to the variable in three scopes:
 - Variable declared in his own scope
-- Variable declared in parent function scope
+- Variable declared in its parent function's scope
 - Variable declared in the global namespace
 
 ```javascript
-var globalVar = "abc";
+var globalVar = "abc"; //Global variable
+
+// Parent self-invoking function
+(function outerFunction (outerArg) { // start of outerFunction's scope
 
-// Parent self invoking function
-(function outerFunction (outerArg) { // begin of scope outerFunction
-  // Variable declared in outerFunction function scope
-  var outerFuncVar = 'x';    
+  var outerFuncVar = 'x'; // Variable declared in outerFunction's function scope   
+  
   // Closure self-invoking function
-  (function innerFunction (innerArg) { // begin of scope innerFunction
-    // variable declared in innerFunction function scope
-    var innerFuncVar = "y";
+  (function innerFunction (innerArg) { // start of innerFunction's scope
+
+    var innerFuncVar = "y"; // variable declared in innerFunction's function scope
     console.log(         
       "outerArg = " + outerArg + "\n" +
       "outerFuncVar = " + outerFuncVar + "\n" +
       "innerArg = " + innerArg + "\n" +
       "innerFuncVar = " + innerFuncVar + "\n" +
       "globalVar = " + globalVar);
-  // end of scope innerFunction
-  })(5); // Pass 5 as parameter
-// end of scope outerFunction
-})(7); // Pass 7 as parameter
+  	
+  // end of innerFunction's scope
+  
+  })(5); // Pass 5 as parameter to our Closure
+
+// end of outerFunction's scope
+
+})(7); // Pass 7 as parameter to the Parent function
 ```
 
-`innerFunction` is closure which is defined inside `outerFunction` and has access to all variable which is declared and defined in outerFunction scope. In addition to this function defined inside the function as closure has access to the variable which is declared in `global namespace`.
+`innerFunction` is a closure which is defined inside `outerFunction` and consequently has access to all the variables which have been declared and defined within `outerFunction`'s scope as well as any variables residing in the program's global scope.
 
-Output of above code would be:
+The output of the code above would be:
 
 ```javascript
 outerArg = 7
@@ -151,8 +157,6 @@ console.log(mul(4)(3)(4)); // output : 48
 ```
 <details><summary><b>Answer</b></summary>
 
-Below is the code followed by the explanation of how it works:
-
 ```javascript
 function mul (x) {
   return function (y) { // anonymous function
@@ -163,14 +167,15 @@ function mul (x) {
 }
 ```
 
-Here the `mul` function accepts the first argument and returns the anonymous function which takes the second parameter and returns the anonymous function which takes the third parameter and returns the multiplication of arguments which is being passed in successive
+Here the `mul` function accepts the first argument and returns an anonymous function which then takes the second parameter and returns one last anonymous function which finally takes the third and final parameter; the last function then multiplies `x`, `y` and `z`, and returns the result of the operation.
+
+In Javascript, a function defined inside another function has access to the outer function's scope and can consequently return, interact with or pass on to other functions, the variables belonging to the scopes that incapsulate it.
 
-In Javascript function defined inside has access to outer function variable and function is the first class object so it can be returned by the function as well and passed as an argument in another function.
 - A function is an instance of the Object type
-- A function can have properties and has a link back to its constructor method
-- A function can be stored as variable
-- A function can be pass as a parameter to another function
-- A function can be returned from another function
+- A function can have properties and has a link to its constructor method
+- A function can be stored as a variable
+- A function can be passed as a parameter to another function
+- A function can be returned by another function
 
 </details>
 

From a14c83391324e6030e3e8ac5f64593e75250a7bc Mon Sep 17 00:00:00 2001
From: Kamlesh Pandey <47499656+KamleshPandey98@users.noreply.github.com>
Date: Thu, 1 Oct 2020 20:03:56 +0530
Subject: [PATCH 42/46] Update README.md

---
 README.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index b8b329b..824b2b1 100644
--- a/README.md
+++ b/README.md
@@ -3776,7 +3776,7 @@ console.log(numb);
 
 </details>
 
-### 4**. What would be the output of following code ?
+### 4. What would be the output of following code ?
 
 ```javascript
 function mul(x){
@@ -3802,7 +3802,7 @@ console.log(mul(2)(3)[1](4));
 
 </details>
 
-### 5**. What would be the output of following code ?
+### 5. What would be the output of following code ?
 
 ```javascript
 function mul(x) {

From 0f3706c1b447b7f012145718d4a259b885bfef89 Mon Sep 17 00:00:00 2001
From: pranav247sw <Pranav.Kulkarni@247software.com>
Date: Mon, 26 Apr 2021 11:49:14 +0530
Subject: [PATCH 43/46] Typp/Grammar checks and corrections

---
 README.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index b8b329b..df5e30f 100644
--- a/README.md
+++ b/README.md
@@ -103,7 +103,7 @@ A closure is a function defined inside another function (called parent function)
 
 The closure has access to the variables in three scopes:
 
-- Variable declared in his own scope
+- Variable declared in its own scope
 - Variable declared in its parent function's scope
 - Variable declared in the global namespace
 
@@ -333,7 +333,7 @@ console.log(output);
 ```
 <details><summary><b>Answer</b></summary>
 
-The code above will output `0` as output. `delete` operator is used to delete a property from an object. Here `x` is not an object it's **local variable**. `delete` operator doesn't affect local variables.
+The code above will output `0` as output. `delete` operator is used to delete a property from an object. Here `x` is not an object, it's a **local variable**. `delete` operator doesn't affect local variables.
 
 
 </details>

From dd5e263b75e971019c8585ea6e2fbe72e9ce254f Mon Sep 17 00:00:00 2001
From: kalon1997 <kalonacharjee@gmail.com>
Date: Thu, 12 May 2022 12:21:33 +0530
Subject: [PATCH 44/46] added 48th ques about callbackhell promise async/await

---
 README.md | 130 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 130 insertions(+)

diff --git a/README.md b/README.md
index fdae8f6..a83a053 100644
--- a/README.md
+++ b/README.md
@@ -2107,6 +2107,136 @@ btn.addEventListener('click', clickHandler.handleClick.bind(clickHandler));
 
 </details>
 
+### 48. How to replace callbackhell with Promise or Async/Await with examples ?
+
+<details><summary><b>Answer</b></summary>
+
+- Part I Callbackhell.
+- Calling one callback function inside another and so on is callbackhell.
+- First we are defining three functions addTen, subFive and mulTwo. 
+- These three functions while called with a number, will return a callback.
+- The callback function will return either result or error.
+
+```js
+const addTen = (num, callback) =>
+  {return callback(num+10, false)}
+```
+
+```js
+const subFive = (num, callback) =>
+  {return callback(num-5, false)}
+```
+
+```js
+const mulTwo = (num, callback) =>
+  {return callback(num*2, false)}
+```
+
+- Now lets call these one by one in nested way.
+- The result of previous will serve as input for next callback.
+
+```js
+const ans = addTen(5, (addRes, addErr) => { // addRess = 15
+        if(!addErr)
+        {
+            return subFive(addRes , (subRes, subErr) => { //subRes = 10
+                if(!subErr){
+                    return mulTwo(subRes, (mulRes, mulErr) => {
+                        if(!mulErr)
+                        {
+                            return mulRes; //20
+                        }
+                    })
+                }
+            })
+        }
+    }) 
+console.log(ans); // 20
+```
+
+- Part II Promise.
+- Promise has two parameters resolve and reject. 
+- Rewrting those three function definations as well, without a callback.
+
+```js
+const addTen = (num) => {return num+10}
+```
+
+```js
+const subFive = (num) => {return num-5}
+```
+
+```js
+const mulTwo = (num) => {return num*2}
+```
+
+- Creating a promise.
+
+```js
+const promise = new Promise((resolve, reject) => {
+    if(true)
+      resolve(5)
+    else
+      reject("Something went wrong ")
+})
+```
+
+- Calling those three functions one by one.
+- "then" will keep on returning the result and if any error "catch" will catch it.
+
+```js
+promise.then(addTen).then(subFive).then(mulTwo).then((ans)=>{
+console.log(ans)
+}).catch((err)=>{console.log(err)});
+```
+
+- Part III Async / Await.
+- It actually uses promise internally.
+
+```js
+const addTen = ( num ) => {
+    return new Promise( ( resolve, reject ) => {
+        resolve( num+10)
+    } )
+}
+```
+
+```js
+const subFive = ( num ) => {
+    return new Promise( ( resolve, reject ) => {
+        resolve( num-5)
+    } )
+}
+```
+
+```js
+const mulTwo = ( num ) => {
+    return new Promise( ( resolve, reject ) => {
+        resolve( num*2)
+    } )
+}
+```
+
+- Put Async keyword before function name and Await before the statments inside the function
+- Await will make the later code wait until the result of that statement is returned.
+- Always put this inside a try/catch block.
+
+```js
+const ans = async (num) => {
+    try {
+        var addRes = await addTen(num);
+        var subRes = await subFive(addRes);
+        var mulRes = await mulTwo(subRes);
+        console.log(mulRes)
+    } catch (err) {
+        console.log(err)
+    }
+}
+ans(5)
+```
+
+</details>
+
 # Coding Questions
 
 ## Passing values by reference vs by value

From fd7a01788d0c33f58ec296e622ae653fb4a45dc6 Mon Sep 17 00:00:00 2001
From: Akhil24-abd <akghnekar2020@gmail.com>
Date: Mon, 17 Oct 2022 23:39:22 +0530
Subject: [PATCH 45/46] Questions added

---
 README.md | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/README.md b/README.md
index fdae8f6..a4fc97a 100644
--- a/README.md
+++ b/README.md
@@ -3910,6 +3910,41 @@ personObj.getName2();
 
 </details>
 
+### 8 . What would be the output of the following code ?
+```javascript
+let a = true;
+let c = 0;
+
+setTimeout(() => {
+	a = false;
+},2000)
+
+while(a){
+	console.log('Hello')
+}
+```
+<details><summary><b>Answer</b></summary> 
+The above program will print Hello infinitely. Since, Javascript is a single threaded language the actual execution happens only on the main thread. So, setTimeout will wailt for 2000 milliseconds on a seperate thread as while loop has occupied the main thread. The exit condition for the loop is to set the variable a as fasle. But as the loop continously running on the main thread , it a cannot be set false.
+</details>
+
+### 9 . What would be the output of the following code ?
+```javascript
+
+let c=0;
+
+let id = setInterval(() => {
+	console.log(c++)
+},200)
+
+setTimeout(() => {
+	clearInterval(id)
+},2000)
+```
+
+<details><summary><b>Answer</b></summary>
+The above program will print 0 to 9 sequentially.
+</details>
+
 ## Contributing
 
 We always appreciate your feedback on how the book can be improved, and more questions can be added. If you think you have some question then please add that and open a pull request. 

From 3bb40311689916c371d8a4fba35cfc13c093b774 Mon Sep 17 00:00:00 2001
From: sumeyra davran <sumeyra.ozgur91@gmail.com>
Date: Sat, 25 Nov 2023 11:37:44 +0300
Subject: [PATCH 46/46] update the answer

---
 README.md | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index a4fc97a..42c4b1f 100644
--- a/README.md
+++ b/README.md
@@ -3120,7 +3120,9 @@ console.log(funcA());
 	
 <details><summary><b>Answer</b></summary>
 
- 1) 
+ 1) funcA  Window {...} 
+    innerFunc1 Window {...} 
+    innerFunA11 Window {...}
 
 </details>