Question 1

What is the difference between var, let, and const?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every scoping and TDZ claim below was actually run in Node — including the classic loop-closure bug. ## 1. Why This Even Matters — A Story First Imagine a shared office whiteboard (`var`) versus a sticky note stuck to one specific desk (`let`/`const`). Anyone in the wh

Question 2

What is the difference between for...of and for...in?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every loop's real output below was actually run in Node. ## 1. Why This Even Matters — A Story First Imagine a numbered coat-check rack (an array) that also has a sticky note taped to its SIDE reading "capacity: 50" — a piece of metadata about the rack itself, not on

Question 3

Describe the concept of 'scope' in JavaScript.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interview questions. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result below was actually run on Node.js v24.19.0 — the printed output is real, captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Picture a program as rooms inside rooms: a house (global scope) contains a kitchen

Question 4

What are `let`, `const`, and `var`? What are their differences?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interview questions. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result below was actually run on Node.js v24.19.0 — the printed output is real, captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Think of three different ways to reserve a locker. `var` reserves a locker for the

Question 5

How do the new Set methods `union()`, `intersection()`, `difference()` and `symmetricDifference()` work?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every operation's real result below was actually run in Node (Baseline-shipped across all major browsers since 2024). ## 1. Why This Even Matters — A Story First Two overlapping guest lists for two different parties: union is the combined list of everyone invited to

Question 6

What is the difference between synchronous and asynchronous code?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals/event-loop interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The exact ordering below was actually run in Node — not recited from a diagram. ## 1. Why This Even Matters — A Story First Ordering food at a counter is synchronous: you stand there, place your order, and nothing else happens until the cashier finishes ri

Question 7

What is the difference between map and forEach?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript array-method and async-pattern interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real return-value proof and the real async-forEach bug demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Handing a stack of letters to a mail carrier who genuinely wa

Question 8

Set methods deep-dive: performance and mutability trade-offs vs manual loops?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript data-structure and performance interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every number shown below is **real, measured output** from actually running the benchmark in this doc on Node.js v24.19.0 — not an assumed percentage. ## 1. Why This Even Matters — A Story First Picture two guest lists for a wedding, each a

Question 9

Implement a robust Event Emitter class.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript design-pattern / low-level API implementation interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every behavior shown below is **real, captured output** from actually running the code in this doc on Node.js v24.19.0, including a direct comparison against Node's actual built-in `node:events` module — not illustrative sample

Question 10

How do you implement a deepFreeze function recursively?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every claim below — including the cycle-safety behavior — was actually run, not just reasoned about. ## 1. Why This Even Matters — A Story First Locking the front door of a house but leaving every internal room door wide open and unlocked technically means "the house

JAVASCRIPTDECODED.

Easy

What is the difference between var, let, and const?

What is the difference between for...of and for...in?

Describe the concept of 'scope' in JavaScript.

What are `let`, `const`, and `var`? What are their differences?

How do the new Set methods `union()`, `intersection()`, `difference()` and `symmetricDifference()` work?

What is the difference between synchronous and asynchronous code?

What is the difference between map and forEach?

Medium

Set methods deep-dive: performance and mutability trade-offs vs manual loops?

Implement a robust Event Emitter class.

How do you implement a deepFreeze function recursively?

What is an IIFE?

How do you handle asynchronous operations in JavaScript?

How would you use ResizeObserver to react to an element's size changing, without polling getBoundingClientRect() on every frame?

Explain prototypal inheritance.

What are closures in JavaScript?

Hard

Explain the event loop, call stack, and task queues.

How does `Atomics.waitAsync()` enable non-blocking waits on SharedArrayBuffer?

Implement an async task queue with a concurrency limit.

What does yield* do in a generator?

What are async iterators and for await...of?

How does SharedArrayBuffer let two Web Workers share memory directly, and what race conditions do you need to guard against?

How would you make a custom class iterable with for...of by implementing Symbol.iterator as a generator method?

How would you implement an async generator function to stream paginated API results, and how does a consumer use it with for await...of?