JavaScript Introduction — Interview Questions & Answers

Ques: What is JavaScript?

Ans: JavaScript (JS) is a high-level, interpreted scripting language used to make web pages interactive, dynamic, and functional.

It works with HTML (structure) and CSS (style) to create complete, user-friendly web experiences.

Ques: Who created JavaScript and when?

• Created by Brendan Eich in 1995 while working at Netscape.
• Originally called LiveScript, later renamed JavaScript.
• It follows the ECMAScript (ES) standard, maintained by ECMA International.

Ques: JavaScript used for?

• Form validation
• Animations and effects
• Dynamic HTML updates (DOM manipulation)
• Event handling (clicks, input, etc.)
• Backend programming using Node.js
• Game development
• Building SPAs (React, Angular, Vue)
• Mobile apps (React Native, Ionic)

Ques: What are the main features of JavaScript?

Feature	Description
Lightweight & Fast	Runs quickly in browsers.
Interpreted Language	No compilation needed.
Cross-Platform	Works on all browsers and OS.
Event-Driven	Responds to user actions.
Object-Based	Uses objects, not pure OOP.
Dynamic Typing	Variables can change types.
Prototype-Based Inheritance	Reuses code using prototypes.
Asynchronous Programming	Handles async code with callbacks, promises, async/await.

Ques: Is JavaScript the same as Java?

Ans: No, they are different languages.

Comparison	JavaScript	Java
Type	Scripting Language	Programming Language
Execution	Browser	JVM (Java Virtual Machine)
Syntax	Loosely typed	Strongly typed
Usage	Frontend & Backend	Backend, Apps
File Extension	.js	.java
Speed	Faster for web tasks	Heavier for web usage

Ques: How can you run JavaScript?

Ans: You can execute JS code in several environments:

1. Browser Console

   • Open DevTools → Console → Type JS code → Press Enter.

2. Inside HTML file

   <script>
     alert("Hello from JS!");
   </script>
   

3. External JS file

   <script src="script.js"></script>
   

4. Node.js Environment

   node app.js
   

Ques: Where can you place JavaScript in HTML?

Ans: You can add JS code in three ways:

Type	Example	Use Case
Inline JS	<button onclick="alert('Hi')">Click</button>	Small, quick actions
Internal JS	<script>console.log('Hello')</script>	Single-page scripts
External JS	<script src="main.js"></script>	Best for large projects

Ques: What are JavaScript Data Outputs?

Ans: JavaScript can display or output data in several ways:

Method	Description	Example
console.log()	Shows output in browser console	console.log("Debug");
alert()	Shows popup box	alert("Hello!");
document.write()	Writes to HTML directly	document.write("Hi!");
innerHTML	Updates specific HTML element	document.getElementById("demo").innerHTML = "Updated!";

Ques: What is the basic syntax of JavaScript?

Ans: JavaScript syntax defines the rules of writing code.

Example:

let name = "Aman";  // variable declaration
const age = 22;     // constant
console.log(name, age);

Ques: What are JavaScript Statements?

Ans: A JavaScript statement is a single line of code that performs an action.

Example:

let a = 5;        // statement 1
let b = 10;       // statement 2
let sum = a + b;  // statement 3
console.log(sum); // statement 4

Ques: What are JavaScript Comments?

Ans: Comments are used to add notes, descriptions, or explanations inside code. They are ignored by the browser during execution.

Types of Comments:

// Single-line comment

/*
 Multi-line comment
 Used to explain large code blocks
*/

Ques: What is the “Cascading” or Execution Order in JS?

Ans: JavaScript executes code line by line (top to bottom) in the order written, unless controlled by functions or conditions.

Example:

console.log("1");
console.log("2");
console.log("3");

Ques: What are the advantages of JavaScript?

Advantages:

• Easy to learn and use.
• Runs directly in the browser (no compiler).
• Improves interactivity.
• Rich ecosystem (Node.js, React, Vue).
• Supported by all browsers.
• Enables full-stack development (frontend + backend).
• Strong community and libraries.

Ques: What are the disadvantages of JavaScript?

Disadvantages:

• Client-side execution → less secure.
• Browser compatibility issues (old browsers).
• Code can be easily viewed and copied.
• Dynamic typing may cause runtime errors.

Ques: What are some popular JavaScript frameworks & libraries?

Frontend Libraries / Frameworks:

• React.js
• Vue.js
• Angular
• Svelte

Backend Frameworks:

• Node.js
• Express.js
• Next.js
• NestJS

Ques: What is ECMAScript?

Ans: ECMAScript (ES) is the official standard that defines the syntax and features of JavaScript.

Each new version (like ES6, ES7, ES8...) adds new features.

Examples:

• ES5 (2009): Strict Mode
• ES6 (2015): let, const, Arrow Functions, Classes
• ES7–ES16: Async/Await, Modules, Optional Chaining, etc.

Ques: What are common mistakes beginners make in JavaScript?

• Forgetting semicolons or {}
• Using undeclared variables
• Mixing var, let, and const carelessly
• Ignoring === vs ==
• Using wrong script placement (in <head> without defer)
• Not checking browser console for errors

Ques: What are some tools used for JavaScript development?

• VS Code – Popular code editor
• Node.js & npm – For server-side JS and package management
• Browser DevTools – For debugging
• ESLint & Prettier – For code quality
• Babel – For ES6+ transpilation
• Webpack / Vite – For bundling and optimization

Ques: Why is JavaScript essential for modern web development?

Because it:

• Controls interactivity and logic in browsers.
• Powers frontend frameworks (React, Angular, Vue).
• Runs on servers (Node.js).
• Connects APIs, databases, and UIs.
• Makes websites dynamic and user-friendly.