🟠 Part 8: 30 Advanced JavaScript Interview Questions and Answers – For Experienced Developers

📌 Meta Description:
Looking to crack advanced JavaScript interviews? Here are 30 real-life, high-level JavaScript questions with simple explanations and examples that every experienced frontend developer should master.

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✅ Introduction:

If you’ve already mastered the JavaScript basics, it's time to level up with advanced topics.

This part covers:

• Closures

• Promises & Async/Await

• Event loop

• Scope chains

• Deep copy vs shallow copy

• Advanced array methods

• Destructuring

• Debouncing & throttling

• this keyword

• And much more…

Each question is written in clear, simple English with practical examples and real-world use cases.

Let’s begin!

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🔶 JavaScript Advanced Interview Questions and Answers

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1. What is a closure in JavaScript?

Answer:
A closure is when an inner function has access to variables from its outer function — even after the outer function has finished running.

function outer() {
  let count = 0;
  return function inner() {
    count++;
    console.log(count);
  }
}
const counter = outer();
counter(); // 1
counter(); // 2

Closures are useful for data privacy and function factories.

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2. What is the event loop in JavaScript?

Answer:
The event loop manages how JavaScript handles asynchronous operations like setTimeout, promises, or API calls — even though JS is single-threaded.

JS places async tasks in a queue, and the event loop processes them after the main stack is clear.

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3. What is the difference between synchronous and asynchronous code?

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4. What are Promises in JavaScript?

Answer:
A Promise is a placeholder for a future value. It can be:

• Pending

• Resolved

• Rejected

let promise = new Promise((resolve, reject) => {
  resolve("Success");
});

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5. What is async/await?

Answer:
async and await allow you to write asynchronous code in a cleaner, readable way — like it’s synchronous.

async function getData() {
  let result = await fetch("api.com/data");
}

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6. What is the difference between deep copy and shallow copy?

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7. What is the difference between call(), apply(), and bind()?

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8. What is the this keyword in JavaScript?

Answer:
this refers to the object calling the function.

const person = {
  name: "Payal",
  greet() {
    console.log(this.name);
  }
}
person.greet(); // Payal

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9. What is the difference between map() and forEach()?

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10. What is a higher-order function?

Answer:
A function that:

• Takes another function as an argument, or

• Returns another function

function greet(callback) {
  callback();
}

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11. What is function currying?

Answer:
Currying is breaking a function with multiple parameters into a chain of functions with one parameter each.

function add(a) {
  return function(b) {
    return a + b;
  }
}
add(2)(3); // 5

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12. Explain memoization.

Answer:
Memoization stores results of expensive function calls to avoid repeating calculations.

let cache = {};
function square(n) {
  if (cache[n]) return cache[n];
  cache[n] = n * n;
  return cache[n];
}

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13. What is the difference between == and === in objects?

Even if two objects have the same content, they are different unless they reference the same memory.

let a = {x: 1};
let b = {x: 1};
a == b; // false

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14. What is event bubbling and event capturing?

• Bubbling: Event goes from target to parent (default).

• Capturing: Event goes from parent to target.

element.addEventListener('click', handler, true); // capture

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15. What are arrow functions and why don’t they have their own this?

Arrow functions don’t bind this. They inherit it from their parent scope.

const obj = {
  name: "Amit",
  greet: () => console.log(this.name) // undefined
};

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16. Explain destructuring in JavaScript.

Answer:
Destructuring allows unpacking values from arrays or objects.

let [a, b] = [1, 2];
let {name, age} = {name: "Amit", age: 30};

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17. What is optional chaining (?.) in JavaScript?

It prevents errors when accessing deeply nested properties.

user?.address?.city;

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18. What is debouncing in JavaScript?

Limits how often a function runs by waiting for a pause.

Used in: search input, resizing.

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19. What is throttling in JavaScript?

Limits a function to run once in a fixed time period, no matter how often it's triggered.

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20. What is setTimeout() and setInterval()?

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21. What is the use of reduce() method?

Answer:
reduce() reduces an array to a single value.

[1, 2, 3].reduce((acc, val) => acc + val, 0); // 6

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22. What is the Temporal Dead Zone (TDZ)?

Answer:
The time between the declaration and initialization of let/const.

Accessing variable before it's initialized = error.

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23. What are generator functions?

They return a generator object and can pause execution using yield.

function* gen() {
  yield 1;
  yield 2;
}

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24. What is the difference between Object.freeze() and Object.seal()?

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25. What is a WeakMap in JavaScript?

Like a map, but keys must be objects and they are garbage collected when not in use.

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26. What is polyfilling in JavaScript?

Answer:
A polyfill is code that adds a feature to older browsers that don’t support it.

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27. What is a module in JavaScript?

Modules help split code into reusable files.

export const add = () => {};
import { add } from './math.js';

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28. What is eval() and why should you avoid it?

eval() runs code from a string. It’s slow and risky (security).

Avoid using it unless absolutely necessary.

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29. Difference between immutability and mutability in JS?

• Mutable: Arrays/objects can be changed.

• Immutable: Strings, numbers, booleans can't be changed after creation.

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30. What are JavaScript design patterns?

Reusable solutions to common problems.

Popular ones include:

• Singleton

• Factory

• Observer

• Module

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🟢 Conclusion:

If you're applying for senior or mid-level JavaScript roles, understanding these advanced concepts is crucial. They're not just theory — you’ll use these in real-world projects every day.

Next, in Part 9, we’ll go even deeper with more expert-level JavaScript questions, including topics like memory leaks, garbage collection, proxies, and performance optimization.

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