add variables and statements

Author: geekysrm

03-variables-and-statements.md

@@ -0,0 +1,59 @@
+## Variables and statements
+
+Semicolons used to end a statement. You can choose to not write them _(because there is ASI: Automatic Semicolon Insertion in Javascript)_.
+
+### Declaring a variable:
+
+`var first = 'Soumya';`
+
+`let first = 'Soumya';`
+
+`const first = 'Soumya';`
+
+(here value is 'Soumya')
+
+-   `let` and `const` were introduced in ES6 (newer).
+    
+-   `var` and `let` can be updated but not `const`.
+    
+    ```javascript
+      var x = 'hey';
+      y = 'hi';
+      
+      let cool = true;
+      cool = false;
+      
+      const age = 10;
+      age = 11; // wrong: throws error
+    
+    ```
+    
+-   In **strict mode**, we have to define a variable first before assigning a value to it.
+    
+    ```javascript
+      dog = 'snickers'; // bad coding, don't do this
+      console.log(dog); // snickers (no error) 
+      
+      'use strict';
+      dog = 'snickers'; // error: dog is not defined
+    
+    ```
+    
+-   If we write `var dog;` dog is _undefined._
+    
+-   **Scoping:**
+    
+    -   **var** : _function scoped_ (only available inside parent functions)
+        
+    -   **let** and **const** : _block scoped_ (available inside a block denoted by _{ }_ )
+        
+-   **Opinion (what to use):** Use `const` by default; if the value of the variable needs to change then use `let`. Almost never use `var`.
+    
+-   **Variable naming conventions:**
+    
+    -   Should not start with capital unless they are a _class_.
+    -   Must start with **a-z** or **_** or **$**.
+    -   If a variable is multi-word, you can use:
+        -   _Camel-case:_ `let iLovePizza = true;`
+        -   _Upper Camel case (in case of classes):_ `ILovePizza`
+        -   _Snake case: `let i_love_pizza=true;`_