added 48th ques about callbackhell promise async/await

Author: Kalon1997

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