Master JavaScript Interviews: Top 20 Must-Know Questions and Answers Revealed!
Master these tricky JavaScript interview questions to ace your next interview.
In today’s fast-paced technology landscape, JavaScript stands as a cornerstone in web development, powering dynamic and interactive user experiences across the globe. Whether you’re a seasoned developer or a newcomer to the field, nailing a JavaScript interview is a crucial step towards advancing your career. “Master JavaScript Interviews: Top 20 Must-Know Questions and Answers Revealed!” is your ultimate guide to preparing for these critical interviews.
Here are the list of JavaScript Interview Question & Answer.
1. What is JavaScript Hoisting?
Hoisting is JavaScript’s default behavior of moving declarations to the top of the current scope (script or function). In JavaScript, a variable can be declared after it has been used. Only declarations are hoisted, not initializations.
Example:
x = 5; // Assign 5 to x
console.log(x); // 5
var x; // Declare x
2. Explain closures in JavaScript.
A closure is a function that has access to its outer function scope even after the outer function has returned. This means a closure can remember and access variables and arguments of its outer function even after the function has finished.
Example:
function outerFunction() {
var outerVar = 'I am outside!';
function innerFunction() {
console.log(outerVar); // Accessing outerVar from innerFunction
}
return innerFunction;
}
var inner = outerFunction();
inner(); // Outputs: 'I am outside!'
3. What is the difference between == and === in JavaScript?
== is the abstract equality operator that compares for equality after type coercion. === is the strict equality operator that compares values and types.
Example:
console.log("5" == 5); // true - type coercion is performed
console.log("5" === 5); // false - different types
4. Explain the event delegation model in JavaScript.
Event delegation is a technique involving adding a single event listener to a parent element that catches all events of a specific type from its child elements. This is useful for handling dynamically added elements or reducing the number of event listeners.
Example:
document.getElementById("parent").addEventListener("click", function(e) {
if (e.target && e.target.nodeName === "LI") {
console.log("List item clicked!");
}
});
5. What are JavaScript Promises?
A Promise in JavaScript is an object representing the eventual completion or failure of an asynchronous operation. It allows you to write asynchronous code in a more synchronous fashion.
Example:
const myPromise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Success!");
}, 1000);
});
myPromise.then((successMessage) => {
console.log("Yay! " + successMessage);
});
6. How do you implement inheritance in JavaScript?
Inheritance in JavaScript can be implemented using prototype chaining, where an object inherits properties and methods from another object.
Example:
function Animal(name) {
this.name = name;
}
Animal.prototype.speak = function() {
console.log(this.name + ' makes a noise.');
}
function Dog(name) {
Animal.call(this, name);
}
Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;
const dog = new Dog('Rex');
dog.speak(); // Rex makes a noise.
7. What are template literals in JavaScript?
Template literals are string literals allowing embedded expressions, multi-line strings, and string interpolation. They are enclosed by backticks (` `).
Example:
const name = 'World';
console.log(`Hello, ${name}!`);
// Output: Hello, World!
8. Explain this keyword in JavaScript.
The this keyword in JavaScript refers to the object it belongs to. It has different values depending on where it is used: in a method, alone, in a function, in an event, etc.
Example:
const person = {
firstName: "Lokesh",
lastName: "Prajapati",
fullName: function() {
return this.firstName + " " + this.lastName;
}
};
console.log(person.fullName()); // Lokesh Prajapati
9. What is a JavaScript callback function?
A callback function is a function passed into another function as an argument and then executed inside the outer function.
Example:
function greeting(name) {
alert('Hello ' + name);
}
function processUserInput(callback) {
var name = prompt('Please enter your name.');
callback(name);
}
processUserInput(greeting);
10. Explain the concept of the Document Object Model (DOM).
The Document Object Model (DOM) is a programming interface for web documents. It represents the page so that programs can change the document structure, style, and content. The DOM represents the document as nodes and objects; that way, programming languages can interact with the page.
Example:
const element = document.getElementById("demo");
element.innerHTML = "Hello World!";
11. What is the difference between var, let, and const?
var is function scoped, let and const are block scoped. var allows re-declaration and updating, let allows updating but not re-declaration, and const neither allows updating nor re-declaration.
Example:
var x = 1;
let y = 2;
const z = 3;
// var can be re-declared and updated
var x = 4;
x = 5;
// let can be updated but not re-declared
y = 6;
// const cannot be updated or re-declared
// z = 7; // Error
// const z = 8; // Error
12. Explain how async and await work in JavaScript.
async and await are extensions of promises. An async function returns a promise, and await is used inside an async function to wait for a promise.
Example:
async function fetchData() {
let response = await fetch('api.example.com/data');
let data = await response.json();
return data;
}
fetchData().then(data => console.log(data));
13. What is the difference between null and undefined in JavaScript?
null is an assignment value, representing no value or no object. undefined means a variable has been declared but has not yet been assigned a value.
Example:
var testVar; // testVar is undefined
console.log(testVar); // outputs: undefined
testVar = null; // testVar is null
console.log(testVar); // outputs: null
14. Explain event bubbling and capturing in JavaScript.
Event bubbling and capturing are two ways of event propagation in the HTML DOM. In bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. In capturing, the event is first captured by the outermost element and then propagated to the inner elements.
Example:
// Event Bubbling
document.getElementById("parent").addEventListener("click", function() {
console.log("Parent clicked");
}, false);
document.getElementById("child").addEventListener("click", function() {
console.log("Child clicked");
}, false);
// Event Capturing
document.getElementById("parent").addEventListener("click", function() {
console.log("Parent clicked");
}, true);
document.getElementById("child").addEventListener("click", function() {
console.log("Child clicked");
}, true);
15. What is the use strict directive in JavaScript?
The use strict directive is a way to opt in to a restricted variant of JavaScript. It helps in writing "safer" JavaScript code by declaring that the code should be executed in "strict mode".
Example:
"use strict";
x = 3.14; // This will cause an error because x is not declared
16. How do JavaScript closures work under the hood?
Closures are a result of JavaScript’s function-level scope and the fact that functions are first-class objects. When a function is declared, it creates a scope. Variables defined in a higher scope can be accessed by functions defined in a lower scope, but the reverse is not true.
Example:
function outer() {
var secret = "secret code";
function inner() {
console.log(secret); // inner can access the outer's variable
}
return inner;
}
var secretFunction = outer();
secretFunction(); // Outputs: "secret code"
17. Explain the difference between synchronous and asynchronous programming in JavaScript.
Synchronous programming executes tasks sequentially. Each task waits for the previous task to finish. Asynchronous programming allows tasks to be performed in the background, not blocking the main execution thread.
Example:
// Synchronous
console.log('First');
console.log('Second');
// Asynchronous
console.log('First');
setTimeout(() => {
console.log('Second');
}, 1000);
console.log('Third');
18. What is an Immediately Invoked Function Expression (IIFE) in JavaScript?
An IIFE is a function that runs as soon as it is defined. It is a design pattern which is also known as a Self-Executing Anonymous Function and contains two major parts. The first is the anonymous function with lexical scope enclosed within the Grouping Operator (). This prevents accessing variables within the IIFE idiom as well as polluting the global scope. The second part creates the immediately invoked function expression () through which the JavaScript engine will directly interpret the function.
Example:
(function() {
var localVar = 'I am local';
console.log(localVar); // Outputs: 'I am local'
})();
console.log(localVar); // Uncaught ReferenceError: localVar is not defined
19. Explain the map method in JavaScript.
The map method creates a new array populated with the results of calling a provided function on every element in the calling array. This is a powerful tool for transforming data in arrays without mutating the original array.
Example:
const numbers = [1, 4, 9];
const roots = numbers.map(Math.sqrt);
// roots is now [1, 2, 3], numbers is still [1, 4, 9]
20. How does prototypal inheritance work in JavaScript?
In JavaScript, prototypal inheritance is a feature in which an object can inherit properties and methods from another object. Traditionally, to get and set the [[Prototype]] of an object, we use Object.getPrototypeOf and Object.setPrototypeOf. Nowadays, in modern language, we use class and extends.
Example:
let animal = {
eats: true
};
let rabbit = {
jumps: true
};
rabbit.__proto__ = animal; // sets rabbit.[[Prototype]] = animal
// we can find both properties in rabbit now:
alert( rabbit.eats ); // true (**)
alert( rabbit.jumps ); // true
Conclusion
In conclusion, these top 20 JavaScript interview questions provide a comprehensive overview of key concepts, functionalities, and features of the JavaScript language that are essential for any JavaScript developer or interviewee to understand.
From fundamental concepts like hoisting, closures, and the differences between == and ===, to more advanced topics such as asynchronous programming with async and await, event delegation, and prototypal inheritance, these questions encompass a wide spectrum of the JavaScript language. Understanding these concepts not only prepares an individual for interviews but also deepens their overall knowledge of JavaScript, making them a more proficient developer.
Questions on newer ES6 features like template literals, let, const, and arrow functions highlight the evolution of JavaScript and its continuous improvement. The discussion on map method, callback functions, and IIFE demonstrates the language's capability to handle arrays, asynchronous operations, and scoping effectively.
Furthermore, understanding the differences between synchronous and asynchronous programming, the intricacies of the this keyword, and JavaScript's unique approach to object-oriented programming through prototypal inheritance are vital for writing efficient and maintainable code.
Additionally, the insights into DOM manipulation, event bubbling and capturing, and strict mode usage reveal the practical aspects of JavaScript in web development, showcasing how JavaScript interacts with HTML and CSS to create dynamic web content.
In essence, mastering these topics not only helps in cracking JavaScript interviews but also lays a strong foundation for building sophisticated web applications, enhancing problem-solving skills, and adapting to the dynamic nature of web development. Whether you are a beginner aiming to enter the field of web development or an experienced programmer seeking to reinforce your understanding of JavaScript, these questions and their answers serve as a valuable resource in your journey of learning and growth in the world of JavaScript.
Stay curious, keep coding, and never stop honing your JavaScript expertise. The more you delve into its intricacies, the more you’ll appreciate its power in shaping the digital world. Happy coding!
We hope you enjoyed this article and found it helpful. If you have any questions, please feel free to leave a comment below.
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