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:

Browser Console
- Open DevTools → Console → Type JS code → Press Enter.

Inside HTML file

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

External JS file
```
<script src="script.js"></script>
```
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.