Interview Questions of JavaScript

Top JavaScript Interview Questions with Answers

Author: Vishal Thakur

Published: August 2021

JavaScript is a powerful and widely-used client-side scripting language. It is essential for web developers to have a strong understanding of JavaScript concepts to excel in interviews. Below are some of the most frequently asked JavaScript interview questions with detailed answers.

1. What is a Higher-Order Function in JavaScript?

Answer: A higher-order function is a function that can accept another function as an argument or return a function as a return value. It is similar to delegates in .NET.

const varfirstOrderFunction = () => console.log('HI!! this is first order'); const varHigherOrderFunction = ReturnFirstOrderFunction => ReturnFirstOrderFunction(); varHigherOrderFunction(varfirstOrderFunction);

2. What is Immediately Invoked Function Expression (IIFE) in JavaScript?

Answer: An IIFE is a JavaScript function that runs as soon as it is defined. It is used to create a private scope and avoid polluting the global namespace.

(function () { // write some code })();

Variables declared inside an IIFE cannot be accessed outside, ensuring data privacy.

3. What is the Difference Between == and === in JavaScript?

Answer: The == operator compares only the value of variables, while === compares both the value and the type of variables.

4. How to Create an Object in JavaScript?

Answer: There are several ways to create objects in JavaScript:

// Object Constructor var myObject = new Object(); // Object Create var myObjectCreate = Object.create(null); // Object Literal var myObjectLitteral = {}; // Functional Constructor function FunctionalConstructor(name) { this.name = name; this.age = 28; } var myObject = new FunctionalConstructor("Vishal Thakur"); // Prototype function FunctionPrototype(){} FunctionPrototype.prototype.name = "Vishal Thakur"; var myObject = new FunctionPrototype(); // ES6 Class class MyClassInitialize { constructor(FullName) { this.FullName = FullName; } } var myObject = new MyClassInitialize("Vishal Thakur"); // Singleton var mySingletonObject = new function(){ this.fullName = "Vishal Thakur"; };

5. What is the Prototype in JavaScript?

Answer: Prototype is a mechanism by which JavaScript objects inherit features from one another. It is a key concept in JavaScript's object-oriented programming.

6. What is the Splice Method in JavaScript?

Answer: The splice() method is used to add or remove elements from an array. It modifies the original array and returns the removed elements.

let firstArray = [1, 2, 3, 4, 5]; let firstVariable = firstArray.splice(0, 2); // returns [1, 2]; original array: [3, 4, 5]

7. What is the Call() Function in JavaScript?

Answer: The call() function is used to invoke a method with a specific this value and arguments provided individually.

var objectOfEmployee1 = {fName: 'Vishal', lName: 'Thakur'}; var objectOfEmployee2 = {fName: 'Johny', lName: 'Cruse'}; function Invoker(obj1, obj2) { console.log(obj1 + ' ' + this.fName + ' ' + this.lName + ', '+ obj2); } Invoker.call(objectOfEmployee1, 'Good Day!!', 'How are you?'); Invoker.call(objectOfEmployee2, 'Good Day!!', 'How are you?');

8. What is the Apply() Function Used For?

Answer: The apply() function is similar to call(), but it takes arguments as an array.

const personalInfo = { name: function(mobile, address) { return this.fName + " " + this.lName + "," + mobile + "," + address; } }; const employee = { fName: "Vishal", lName: "Thakur" }; personalInfo.name.apply(employee, ["1234567890", "Indore India"]);

9. What is the Bind() Method Used For?

Answer: The bind() method creates a new function with a fixed this value. It is useful for creating functions with a specific context.

var firstEmployeeDetails = {fullName: 'Vishal Thakur', mobile: '1234567890'}; var secondEmployeeDetails = {fullName: 'Avtrit Thakur', mobile: '0912345689'}; function invoker(obj1, obj2) { console.log(obj1 + ' ' + this.fullName + ' ,' + this.mobile + ', '+ obj2); } var varDetailsEmployee1 = invoker.bind(firstEmployeeDetails); var varDetailsEmployee2 = invoker.bind(secondEmployeeDetails); varDetailsEmployee1('Hello', 'Hope you are doing great!!'); varDetailsEmployee2('Hi', 'Have a great day!!');

10. What is the Difference Between Splice() and Slice() Methods?

Answer: The splice() method modifies the original array by adding or removing elements, while slice() returns a new array with selected elements without modifying the original array.

11. What is the Arrow Function in JavaScript?

Answer: Arrow functions are a concise way to write function expressions in JavaScript. They have a shorter syntax and do not have their own this context.

const myFunction = () => { //do some line of code here };

12. What is Hoisting in JavaScript?

Answer: Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their scope during the compilation phase. However, only declarations are hoisted, not initializations.

var objectMessage; console.log(objectMessage); // undefined objectMessage = 'The Message variable has been hoisted at the top';

13. What is a Service Worker in JavaScript?

Answer: A service worker is a script that runs in the background, separate from web pages, and enables features like offline support and push notifications.

14. What is Web Storage in JavaScript?

Answer: Web storage allows storing data in the browser in key/value pairs. It has two types:

• Local Storage: Stores data with no expiration date.
• Session Storage: Stores data for one session and clears it when the browser is closed.

15. What are the Three Main Parts of JavaScript?

Answer: JavaScript consists of three main parts:

1. ECMAScript (the core language)
2. JavaScript Engine (e.g., V8 in Chrome)
3. JavaScript Runtime (e.g., Node.js, browsers)

Conclusion

These are some of the most commonly asked JavaScript interview questions. Understanding these concepts will help you prepare for your next JavaScript interview. If you found this guide helpful, feel free to leave a comment below!

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