Top 10 JavaScript Interview Questions You Must See Before Your Interview

JavaScript is an essential cornerstone of modern web development. Whether you are building a simple website or a complex web application, JavaScript provides the functionality needed to make your projects interactive and dynamic. This article will prepare you for JavaScript interviews by covering the top 10 most common questions, complete with detailed answers.

What is JavaScript?

JavaScript is a high-level, interpreted programming language integral to creating interactive web pages. It allows developers to implement complex features on web pages, from displaying timely content updates to interactive maps, animated graphics, and much more. Initially developed for front-end development, JavaScript's versatility extends to back-end development with environments like Node.js.

JavaScript Data Types

Understanding data types in JavaScript is fundamental. JavaScript includes six primitive data types and one complex type.

Primitive Data Types

• String: Represents textual data. Example: "Hello, World!"
• Number: Represents both integer and floating-point numbers. Example: 42, 3.14
• Boolean: Represents actual or false values. Example: true, false
• Null: Represents the intentional absence of any object value. Example: null
• Undefined: Indicates a variable that has not been assigned a value. Example: let x;
• Symbol: Represents a unique identifier. Example: Symbol('description')

Object Data Type

Objects in JavaScript are collections of properties. This includes more complex data types like arrays and functions.

• Arrays: Ordered collections of values. Example: [1, 2, 3]
• Functions: Blocks of code designed to perform particular tasks. Example: function greet() { return "Hello"; }

Difference Between == and ===

The == and === operators are used for comparison but function differently.

• == (Equality Operator): Compares values for equality, performing type conversion if necessary. Example: 5 == "5" returns true.
• === (Strict Equality Operator): Compares value and type without performing type conversion. Example: 5 === "5" returns false.

Understanding Closures

A closure in JavaScript is a function that retains access to its lexical scope, even when the function is executed outside that scope. This feature is particularly useful for data encapsulation and creating private variables.

Example:

function outerFunction() {

    let outerVariable = 'I am outside!';

    function innerFunction() {

        console.log(outerVariable);

    }

    return innerFunction;

}

const myClosure = outerFunction();

myClosure(); // Logs: I am outside!

The 'this' Keyword

The this keyword refers to the context in which a function is executed. Its value depends on how the function is called.

• Global Context: In the global execution context, this refers to the global object.
• Object Method: When used in a method, this refers to the object the method is called on.
• Constructor Function: In a constructor function, this refers to the newly created instance.

Callbacks in JavaScript

A callback is a function passed into another function as an argument, which is then invoked inside the outer function to complete some action.

Example:

function fetchData(callback) {

    // Simulating an asynchronous operation

    setTimeout(() => {

        callback('Data received');

    }, 1000);

}

function displayData(data) {

    console.log(data);

}

fetchData(displayData); // Logs: Data received

Arrow Functions

Arrow functions provide a more concise syntax for writing function expressions and do not have their own this context, inheriting this from the parent scope.

Syntax and Example:

const add = (a, b) => a + b;

console.log(add(2, 3)); // Logs: 5

Event Bubbling

Event bubbling is a type of event propagation in the DOM where an event triggered on a child element bubbles up to its parent elements, triggering the event handlers of each parent.

Example:

<div id="parent">

    <button id="child">Click me</button>

</div>

<script>

    document.getElementById('parent').addEventListener('click', () => {

        console.log('Parent clicked');

    });

    document.getElementById('child').addEventListener('click', () => {

        console.log('Child clicked');

    });

</script>

Clicking the button will log "Child clicked" followed by "Parent clicked".

let vs. var

The let and var keywords are used to declare variables, but they differ in scope and hoisting behavior.

• let: Block-scoped, not hoisted to the top of the block.
• var: Function-scoped, hoisted to the top of the function.

Example:

if (true) {

    let blockScoped = 'I am block scoped';

    var functionScoped = 'I am function scoped';

}

console.log(functionScoped); // Logs: I am function scoped

console.log(blockScoped); // ReferenceError: blockScoped is not defined

Prototypal Inheritance

JavaScript uses prototypal inheritance, meaning objects inherit properties and methods from other objects via the prototype chain.

Example:

function Animal(name) {

    this.name = name;

}

Animal.prototype.speak = function() {

    console.log(`${this.name} makes a noise.`);

};

const dog = new Animal('Dog');

dog.speak(); // Logs: Dog makes a noise.

What is the main difference between JavaScript and other programming languages?

JavaScript is primarily a client-side scripting language used to create interactive effects within web browsers, whereas many other languages are often used for server-side development.

How can I debug JavaScript code?

JavaScript code can be debugged using browser developer tools, console logging, and debugging tools like Chrome DevTools.

What are some common frameworks and libraries in JavaScript?

Popular JavaScript frameworks and libraries include React, Angular, Vue.js, and jQuery.

How does JavaScript handle asynchronous operations?

JavaScript handles asynchronous operations using callbacks, promises, and async/await syntax.

What is the future of JavaScript in web development?

The future of JavaScript looks promising with continuous improvements and new frameworks/libraries enhancing its capabilities in both front-end and back-end development.

Conclusion

Mastering JavaScript is crucial for any web developer, and understanding these fundamental concepts will prepare you for any interview. From closures to prototypal inheritance, each topic plays a vital role in creating efficient, dynamic web applications.