Question 1

How does 'use strict' change what `this` refers to inside a regular function called without a receiver, compared to non-strict mode?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript this-binding interview questions. **Difficulty:** Medium > **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 — captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Imagine handing someone a form that says "sign here as: ___" with the blank left empty. In a lenient office,

Question 2

What is hoisting 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 Think of a scope like a conference room with name tags laid out before the meeting

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 the quirks of the typeof operator?

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 result below, including the real thrown error, was actually run in Node. ## 1. Why This Even Matters — A Story First Asking "does this specific book exist anywhere in the library?" is a genuinely safe question to ask even if the book's title was never catalogued

Question 5

What is JSON and why is it commonly used?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals and data-interchange 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 Imagine shipping a package overseas. A JS backend, a Python se

Question 6

How do you implement infinite currying in JavaScript?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript closures and functional-programming interview questions. **Difficulty:** Medium > **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 — captured output, not illustrative sample text. ## 1. Why This Even Matters — A Story First Think of a vending machine that keeps a running tab open: each coin you insert (each f

Question 7

Explain 'this' keyword in JavaScript.

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interview questions. **Difficulty:** Medium > **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. This doc covers the CONCEPT of this (why it behaves the way it does); for the exact binding-rule priority order with verified o

Question 8

What is memoization?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals / performance interview questions. **Difficulty:** Easy-Medium > **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 student working through a long worksheet of math pro

Question 9

What are modules in JavaScript?

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:. The module-caching claim below was actually run and confirmed inside a real CommonJS script. ## 1. Why This Even Matters — A Story First A shared office supply closet, restocked once at the start of the day, is used identically by every single department that visits it

Question 10

What is the difference between call, apply, and bind?

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 timing and this-binding claim below was actually run in Node. ## 1. Why This Even Matters — A Story First Think of `this` as a name tag a function wears while it runs. `call` and `apply` are like handing someone a name tag and asking them to introduce themselve

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Easy

How does 'use strict' change what `this` refers to inside a regular function called without a receiver, compared to non-strict mode?

What is hoisting in JavaScript?

Describe the concept of 'scope' in JavaScript.

What are the quirks of the typeof operator?

What is JSON and why is it commonly used?

Medium

How do you implement infinite currying in JavaScript?

Explain 'this' keyword in JavaScript.

What is memoization?

What are modules in JavaScript?

What is the difference between call, apply, and bind?

How does the 'this' keyword work?

What is the difference between `Object.groupBy()` and `Map.groupBy()` — keys, prototypes and ordering?

What is the difference between arrow and regular functions?

Hard

What are function composition and pipe?

Why can pushing a single non-numeric value into a large numeric array tank its performance in V8, and how would you avoid it?

Implement an async task queue with a concurrency limit.

What is the Reflect API?

What is currying?

What does the 'new' keyword do?

How would you implement a curry() function that supports partial application with placeholder arguments?

What is a Proxy in JavaScript?