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?

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

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?

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