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

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 3

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 4

What is globalThis?

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 identity claim below was actually run in Node. ## 1. Why This Even Matters — A Story First Imagine three different countries calling the same building "City Hall," "Rathaus," and "Ayuntamiento" — the same physical building, three different local names, and any trav

Question 5

How does instanceof work?

Accepted Answer

**Target Audience:** Engineers preparing for precise JavaScript object-model 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. instanceof is built directly on the prototype chain covered in

Question 6

What is `Error.isError()` and why is `instanceof Error` unreliable across realms?

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:. The cross-realm failure below was actually reproduced using Node's own real `vm` module — not merely described. ## 1. Why This Even Matters — A Story First Two separate countries can each mint their own, genuinely different "$20 bill" — both are real, valid currency

Question 7

How do you correctly check for NaN?

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 coercion and comparison result below was actually run in Node. ## 1. Why This Even Matters — A Story First Asking a bouncer "is this person over 21?" and getting "yes" back sounds definitive — until you learn the bouncer first tried to convert the person's PET in

Question 8

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 9

What is lexical scoping and the scope chain?

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 GENERAL rule for how any variable lookup resolves in JavaScript; for the specific case where a function out

Question 10

What is an IIFE?

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 scoping and immediate-execution claims below were actually run in Node. ## 1. Why This Even Matters — A Story First Think of a magician who steps behind a curtain, does some private setup work, and immediately steps back out — the audience never sees, and can never

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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?

Describe the concept of 'scope' in JavaScript.

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

What is globalThis?

How does instanceof work?

What is `Error.isError()` and why is `instanceof Error` unreliable across realms?

Medium

How do you correctly check for NaN?

What are modules in JavaScript?

What is lexical scoping and the scope chain?

What is an IIFE?

How does the 'this' keyword work?

What are the basics of regular expressions in JavaScript?

What does `using` / `await using` do for Explicit Resource Management (ES2024)?

What does 'use strict' do?

Hard

What are Symbols and what are they used for?

What is ShadowRealm and how does it provide true JavaScript sandboxing?

How do you implement a simple Promise library from scratch?