JavaScript Advanced Concepts — Interview Questions & Answers

Ques: What are advanced JavaScript concepts?

Ans: Advanced JavaScript concepts go beyond basic syntax and cover topics like Strict Mode, Modules, Closures, Asynchronous programming, and Debugging techniques — all essential for writing efficient, scalable, and secure JavaScript code.

Ques: Why is learning advanced JavaScript important?

Ans: Because modern web apps require:

• Optimized and maintainable code
• Modular structure (Modules)
• Strict and secure coding (Strict Mode)
• Effective error detection (Debugging)
• Compatibility across browsers

Ques: Name a few advanced JavaScript topics developers must know.

• Closures & Scope
• Prototype & Inheritance
• Modules (import/export)
• Asynchronous JS (Promises, Async/Await)
• Strict Mode
• Event Loop
• Debugging techniques

Ques: What is Strict Mode in JavaScript?

Ans: Strict Mode is a restricted variant of JavaScript introduced in ES5 that helps catch common coding errors and unsafe actions early.

It’s enabled by adding "use strict"; at the beginning of a script or function.

"use strict";
x = 10; //  Error: x is not defined

Ques: How do you enable Strict Mode?

Ans: You can enable it:-

For the entire script:

"use strict";

For a single function:

function test() {
  "use strict";
  // code here
}

Ques: What are the benefits of using Strict Mode?

• Prevents the use of undeclared variables
• Makes debugging easier
• Eliminates silent errors
• Disables dangerous features (like with statement)
• Prevents duplicate parameter names

Ques: What happens if you use undeclared variables in Strict Mode?

Ans: It throws a ReferenceError instead of creating a global variable automatically.

"use strict";
x = 5; //  ReferenceError

Ques: Can you use Strict Mode in ES modules by default?

Ans: Yes, All ES Modules (import / export) automatically run in Strict Mode.

Ques: What is not allowed in Strict Mode?

• Using undeclared variables
• Deleting variables or functions
• Duplicating parameter names
• Writing to read-only properties
• Using octal literals (like 010)
• Using with statements

Ques: Example showing Strict Mode catching silent errors:

"use strict";
delete Object.prototype; //  TypeError

Ques: What are JavaScript Modules?

Ans: Modules are independent files that encapsulate code and export or import functionalities.

They help organize JavaScript projects into reusable components.

Ques: Why use Modules?

• Keeps code organized
• Avoids global variable conflicts
• Improves maintainability
• Enables reuse across projects

Ques: How do you export code from a module?

Ans: You can use named or default exports.

// named export
export const name = "JavaScript";

// default export
export default function greet() {
  console.log("Hello JS!");
}

Ques: How do you import a module in JavaScript?

import { name } from "./file.js";
import greet from "./file.js";

Ques: Can you rename imports?

Ans: Yes.

import { name as lang } from "./file.js";

Ques: What is the difference between Named Export and Default Export?

Feature	Named Export	Default Export
Exported Name	Must match during import	Can use any name
Number per file	Many	Only one
Syntax	export {}	export default

Ques: What is the difference between ES Modules and CommonJS?

Feature	ES Modules	CommonJS
Used In	Browser, Node.js (new)	Node.js (old)
Syntax	import/export	require/module.exports
Loading	Static	Dynamic

Ques: Example of mixing imports and exports:

// utils.js
export const PI = 3.14;
export default function area(r) {
  return PI * r * r;
}

// main.js
import area, { PI } from "./utils.js";
console.log(area(5));

Ques: Can modules run without a web server?

Ans: No, You must run modules through a server environment (like http-server) or frameworks (like React, Vite, or Node.js with "type": "module").

Ques: Are modules in JavaScript automatically in Strict Mode?

Ans: Yes, All ES6 modules automatically run in Strict Mode — no need to add "use strict" manually.

Ques: What is debugging?

Ans: Debugging is the process of finding and fixing errors (bugs) in your JavaScript code using tools like the browser console or debugger keyword.

Ques: What are common debugging tools in JavaScript?

• Console API (console.log, console.error, console.table)
• Debugger keyword
• Browser Developer Tools (F12)
• Breakpoints & Watch expressions
• Performance profiler

Ques: What is the use of console.log()?

Ans: It prints information to the browser console — commonly used for quick debugging.

console.log("Value of x:", x);

Ques: What are some other useful console methods?

Method	Description
console.warn()	Displays a warning message
console.error()	Displays an error message
console.table()	Displays tabular data
console.group()	Creates grouped logs
console.time() / console.timeEnd()	Measures performance

Ques: What is the debugger keyword used for?

Ans: When the browser encounters debugger;, it pauses code execution and allows inspection of variables and call stack.

function test(num) {
  debugger; // pause here
  return num * 2;
}
test(10);

Ques: How do you add breakpoints in JavaScript?

Ans: Open DevTools → Sources tab → Click line number → Adds a breakpoint.

When the code runs, it pauses at that line.

Ques: What are watch expressions in debugging?

Ans: They let you monitor specific variables or expressions while the code runs, helping trace their changing values.

Ques: What is step-by-step debugging?

Ans: You can step through code line by line using:

• Step Into: Go inside a function
• Step Over: Skip function body
• Step Out: Return from current function

Ques: How can you debug asynchronous code (Promises / async-await)?

• Use console.log inside .then() or await sections
• Use breakpoints inside async functions
• Chrome DevTools now supports async call stacks

Ques: What are best practices for debugging JS efficiently?

• Use meaningful console logs
• Avoid leaving console.log in production
• Use breakpoints for complex code
• Group logs (console.group)
• Test with small inputs
• Understand stack traces