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JavaScript Handbook

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JavaScript Introduction — Top Interview Q&A

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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?

FeatureDescription
Lightweight & FastRuns quickly in browsers.
Interpreted LanguageNo compilation needed.
Cross-PlatformWorks on all browsers and OS.
Event-DrivenResponds to user actions.
Object-BasedUses objects, not pure OOP.
Dynamic TypingVariables can change types.
Prototype-Based InheritanceReuses code using prototypes.
Asynchronous ProgrammingHandles async code with callbacks, promises, async/await.

Ques: Is JavaScript the same as Java?

Ans: No, they are different languages.

ComparisonJavaScriptJava
TypeScripting LanguageProgramming Language
ExecutionBrowserJVM (Java Virtual Machine)
SyntaxLoosely typedStrongly typed
UsageFrontend & BackendBackend, Apps
File Extension.js.java
SpeedFaster for web tasksHeavier 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:

TypeExampleUse 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:

MethodDescriptionExample
console.log()Shows output in browser consoleconsole.log("Debug");
alert()Shows popup boxalert("Hello!");
document.write()Writes to HTML directlydocument.write("Hi!");
innerHTMLUpdates specific HTML elementdocument.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.

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