🎯 JS Interview Checklist - Part 2 (Advanced)

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Listen to this article

You have covered the basics ✅

Now, you are ready to impress the interviewer with some advanced concepts 👨‍🔬

Let's get started.

If you haven't read Part-1: rajatgupta.xyz/js-interview-1

📚 Polyfills

❓ Most commonly asked: map, bind

A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it.

  • Let's implement it for map
// this - array
// this[i] - current value
Array.prototype.myMap = function (cb) {
  var arr = []
  for (var i = 0; i < this.length; i++) {
    arr.push(cb(this[i], i, this))
  return arr

const arr = [1, 2, 3]
console.log(arr.myMap((a) => a * 2)) // [2, 4, 6]
  • bind
let name = {
  first: 'Rajat',
  last: 'Gupta',
let display = function () {
  console.log(`${this.first} ${this.last}`)

Function.prototype.myBind = function (...args) {
  // this -> display
  let obj = this
  return function () {

// let displayMe = display.bind(name)
let displayMe = display.myBind(name)

displayMe() // Rajat Gupta

But this is the basic implementation, suppose we have parameters in our display & displayMe function

let display = function (city, country) {
  console.log(`${this.first} ${this.last} from ${city}, ${country}`)

Function.prototype.myBind = function (...args) {
  let obj = this
  // get the args except the first one
  params = args.slice(1)
  return function (...args2) {
    obj.apply(args[0], [...params, ...args2])

let displayMe = display.myBind(name, 'Delhi')
displayMe('India') // Rajat Gupta from Delhi, India

➰ Event Loop

A very ❗important topic to understand asynchronous behavior.

Instead of providing a half-baked explanation here, I recommend watching this video if you haven't already:

🔒 Closures ❗important

❓ Explain, output Q, advantages, disadvantages

You have probably used it without even realizing it.

This section will have a lot of fancy words - so bear with me. We will cover them one by one.

Function bundled together with its lexical environment forms a closure

Okay, what is lexical environment❓

It is essentially the surrounding state - the local memory along with the lexical environment of its parent.

Whaaat? 🤯 I know it's a bit of a doozy. Let's understand it with an example.

function x() {
  var a = 7
  function y() {
  return y

var z = x()
console.log(z) // [Function: y]

When x is invoked, y is returned. Now, y is waiting to be executed. Kind of like a loaded gun waiting to be shot! 🔫

So, when we finally invoke z - y is invoked. Now, y has to log a so it first tries to find 🔍 it in the local memory but it's not there. It goes to its parent function. It finds a there.

Voila❗There you have it - this is closure.

Even when functions are returned (in the above case y) they still remember their lexical scope (where it came from)

Totally, unrelated quote for kicks 👻:

They may forget what you said - but they will never forget how you made them feel - Carl W. Buehner

I swear the rest of the article is legit 🤞 Keep reading.

Advantages 😎

  • Currying
let add = function (x) {
  return function (y) {
    console.log(x + y)

let addByTwo = add(2)
  • Data Hiding/Encapsulation

Suppose, you want to create a counter application. Every time, you call it - the count increases by 1. But you don't want to expose the variable outside the function. How to do it?

You guessed it - closure❗

function Counter() {
  var count = 0
  this.incrementCount = function () {

console.log(count) // Error: count is not defined
var adder = new Counter()
adder.incrementCount() // 1

Disadvantages 😅

  • Overconsumption of memory or memory leaks can happen.

For example, the closed-over-variable will not be garbage collected. As even if the outer function has run, the returned inner function still has a reference to the closed-over-variable.

Note: Garbage collector basically removes unused variables from the memory automatically.

🤝Promises ❗important

❓ Syntax, explanation, output Q

Again, a very very important topic.

The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

It is in one of these three states:

  • pending: initial state, neither fulfilled nor rejected
  • fulfilled: operation was completed successfully
  • rejected: operation failed
const promise = new Promise((resolve, reject) => {
  let value = true;
  if (value) {
    resolve("hey value is true");
  } else {
    reject("there was an error, value is false");

  .then((x) => {
  .catch((err) => console.log(err));

Note: resolve and reject are just conventional names. Call it pizza🍕 if you like

Instead of then/catch - we can also use async/await

async function asyncCall() {
  const result = await promise


One of the advantages of promises is that it is a much cleaner syntax. Earlier, it used to be a callback hell 🌋

Now now, there is a lot more than this to Promises. Covering it in depth will require a separate blog post. Let me know in the comments if you want that.

👪 Prototypes, Prototypal Inheritance

❓ Syntax, explanation

You may have noticed the keyword prototype from the polyfill example.

From the emoji - you might think this is like the inheritance in other languages like C++. But it's not.

Whenever we create anything (object, function) in JS - JS Engine automatically attaches that anything with some properties and methods

All this comes via prototypes

__proto__ is the object where JS is putting it all

Let's take some examples. Fire up your consoles!

let arr = ['Rajat', 'Raj']
console.log(arr.__proto__) // same as Array.prototype
console.log(arr.__proto__.__proto__) // same as Object.prototype
console.log(arr.__proto__.__proto__.__proto__) // null

All this is called a prototype chain

We can do the same with objects and functions as well.

We will always find Object.prototype behind the scenes. That's why you may have heard - everything in JS is nothing but an object 🤯

Prototypal Inheritance

let object = {
  name: 'Rajat',
  city: 'Delhi',
  getIntro: function () {
    console.log(`${this.name}, ${this.city}`)

let object2 = {
  name: 'Aditya',

Note: Don't modify prototypes this way. It's just for understanding. The right way to do it: javascript.plainenglish.io/how-prototypal-i..

object2.__proto__ = object

By doing this, object2 gets access to the object's properties. So, now we can do:


This is prototypal inheritance.

Coming back to the polyfill of bind from before:

Function.prototype.myBind = function () {
  // code
// now we can access this method whenever we create a new function
function a(){
 // code

console.log(a.__proto__) // will have myBind method

⚡Performance Optimization

  • Debouncing
  • Throttling
  • Caching
  • Code Splitting
  • Bundling, Minification
  • Server-side rendering

Note: Generally not asked to freshers 👶 but good to know 📝

Let's cover debouncing and throttling.


Another favorite of interviewers.

Let's understand it by creating a search bar.

Demo: codesandbox.io/s/debounce-input-field-o5gml

Create a simple input field in index.html

<input type="text" id="text" />

Now, index.js. Don't forget to add it to index.html

const getData = (e) => {
const inputField = document.getElementById("text");

const debounce = function (fn, delay) {
  let timer;
  return function () {
    let context = this;
    timer = setTimeout(() => {
      fn.apply(context, arguments);
    }, delay);

inputField.addEventListener("keyup", debounce(getData, 300));

First, we have selected the input and added an event listener to it. Then we created a debounce function which takes a callback function and delay.

Now, inside the debounce function we create a timer using setTimeout. Now, this timer's job is to make sure - the next call for getData only happens after 300 ms. This is what debouncing is.

Also, we use clearTimeout to remove it. Don't want too many of them hanging out there taking up memory space!

Phew! Lots of theory. Let's do a fun challenge. You must have seen the countdown before a game starts (it goes like 10, 9, 8, .... with some delay in between). Try to write a program for it.


let count = 10

for (let i = 0; i < 10; i++) {
  function timer(i) {
    setTimeout(() => {
    }, i * 500)

Were you able to solve it? Did it differently? Let me know your solution.

🛑 Throttling

Let's take an example again. Suppose, on every window resize event - we call an expensive function. Now, we want it such that the expensive function will only be executed once in the given time interval. This is what throttling is.

Create anindex.html and an index.js with the following code:

const expensive = () => {

const throttle = (fn, limit) => {
  let context = this
  let flag = true
  return function () {
    if (flag) {
      fn.apply(context, arguments)
      flag = false
    setTimeout(() => {
      flag = true
    }, limit)
const func = throttle(expensive, 2000)
window.addEventListener('resize', func)

Almost the same as debouncing. The key difference is the flag variable. Only, when it's true - we are invoking the callback function. And it is set to true inside the setTimeout . So the value is true only after the desired time limit.

So, what's the difference between debounce and throttling❓

Let's take the search bar 🔍 example from above. When we are debouncing the input field - we are saying that only fetch the data when the difference between two keyup events is at least 300 ms.

In the case of throttling, we make a function call only after a certain period of time. Suppose, you are searching for an encyclopedia in the search bar. Say, the first call is made on e and it took us 300 ms to reach p. The next call will be made then only. All the events in between will be ignored.

So, to summarize, debouncing - when the difference between two keyup events is 300 ms and throttling - when the difference between two function calls is 300 ms. Basically, the function is called after a certain interval of time.

And we are doneee 🏁

I hope this article was useful. Do like, share, and comment 👇

Until next time 👋

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Anand Baraik's photo

Very well written article.

David S D's photo

Wow. One of the best, "to the point" articles ever!

Rajat Gupta's photo

Glad you liked it!