+100 Common JavaScript Developer Interview Questions and Answers (Part 1)

Study these questions to make your interview easier.

At present, JavaScript has become the most used language as of 2022. Google, Meta, and other FAANG companies use JavaScript to build complex web applications. They used it to create frameworks like Angular which is created by Google using JavaScript, and React.Js created by Meta(Facebook) which is created by using JavaScript.So JavaScript is now the most recommend language to learn in 2022 and I think it will continue to be most recommended even in the coming years. In the next link, you see the most-used programming languages among developers worldwide as of 2022.

Most used programming languages among developers worldwide as of 2022

JavaScript

So, if you are planning to start learning web development, I recommend learning JavaScript, now is the right time to dive in. And in this article, I will write +100 common JavaScript questions I faced in my previous interviews.

1- What is the first class function in JavaScript?

Functions are first-class objects, which means functions in JavaScript are treated like any other variable. The functions in JavaScript can be passed as an argument to other functions, it can be returned by another function and assigned as a value to a variable.

// Assign the function to variable
const Fn = function() {}
// Pass a functions as arguments into functions and call it
const Fn2 = function(Func){
 Func();
}
// return functions as value from another function
const Fn3 = function(){
 return function(){
   console.log('hello from another function');
 }
}

2- What is the difference between slice and splice?

One of the questions that cause some confusion even for senior developers.

Slice:

• Doesn’t modify the original array, which means it’s “immutable”
• Returns the subset of the original array.
• Used to pick the elements from the array.

Splice

• You can modify the original array, which means it’s (mutable).
• Returns the deleted elements as an array.
• Used to insert or delete elements to/from the array.

3- What is JSON and its common operations?

JSON is a text-based data format following JavaScript object syntax, which was popularized by Douglas Crockford. It is useful when you want to transmit data across a network and it is basically just a text file with an extension of .json, and a MIME type of application/json.

Parsing: Converting a string to a native object

Stringification: converting a native object to a string so it can be transmitted across the network.

4- What is a higher-order function?

A higher-order function is a function that accepts another function as an argument or returns a function as a return value or both.

const firstOrderFunc = () =>
  console.log("Hello, I am a First order function");
const higherOrder = (ReturnFirstOrderFunc) => 
  Return FirstOrderFunc();
higherOrder(firstOrderFunc);

5- What is the difference between let and var?

var:

• It is been available from the beginning of JavaScript
• It has a function scope
• Variables will be hoisted

let:

• Introduced as part of ES6
• It has block scope.
• Hoisted but not initialized.

let’s take a snippet of code to see the difference between them:

function userDetails(username) {
  if (username) {
    console.log(salary); // undefined due to hoisting
    console.log(age); // ReferenceError: Cannot access 'age' before  initialization.
    let age = 30;
    var salary = 10000;
  }
  console.log(salary); //10000 (accessible to due function scope)
  console.log(age); //error: age is not defined(due to block scope)
}
userDetails("Hassanien");

6- What are modules?

Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor.

Part 2