Question 1

How does Array.prototype.toSorted() differ from .sort(), and why does that matter when updating state immutably?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript immutable-state-update 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 Picture a manager who wants to keep a snapshot of yesterday's inventory list before making changes

Question 2

How does Object.groupBy() work in ES2024 and when should you use it over reduce?

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 grouping result and prototype check below was actually run in Node (v21+ required; this project verified on v24.19.0). ## 1. Why This Even Matters — A Story First A grouping bucket labeled with a plain sticky note is a regular object — perfectly fine, but it si

Question 3

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 4

What is the difference between includes and indexOf?

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 comparison below, including the NaN case, was actually run in Node. ## 1. Why This Even Matters — A Story First Imagine asking two different assistants "is there a red car in this parking lot?" One (`indexOf`) answers with a parking spot number, or `-1` if there

Question 5

How would you use the Page Visibility API to pause a setInterval-based poller when a user switches to a different browser tab?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript browser-API 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 against a real jsdom document 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 delivery app polling a restaurant every fe

Question 6

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 7

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 8

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 9

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 10

How do you convert a callback-based function to a Promise?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/Node.js fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every resolve/reject path below was actually run in Node, against a real (simulated) legacy callback function. ## 1. Why This Even Matters — A Story First A legacy callback-based function is like a delivery service that calls you back on the phone when your p

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Easy

How does Array.prototype.toSorted() differ from .sort(), and why does that matter when updating state immutably?

How does Object.groupBy() work in ES2024 and when should you use it over reduce?

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

What is the difference between includes and indexOf?

How would you use the Page Visibility API to pause a setInterval-based poller when a user switches to a different browser tab?

How does instanceof work?

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

What is the difference between map and forEach?

Medium

How do you implement infinite currying in JavaScript?

How do you convert a callback-based function to a Promise?

Explain 'this' keyword in JavaScript.

When should you use Map and Set over objects and arrays?

Implement a robust Event Emitter class.

Explain event delegation.

How does Array.reduce work?

How do you handle asynchronous operations in JavaScript?

Hard

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?

How would you use the new Iterator helper methods (.map, .filter, .take, .drop) to process a large or infinite iterator lazily?

How do Stage 3 JavaScript Decorators work — class, method and field decorators?

What are tagged template literals?

How would you write a deep-clone function that correctly handles a self-referencing (circular) object without infinite recursion?

How do you combine Stage 3 decorators with `using` for auto-disposable resources?

How would you implement a throttle function with configurable leading- and trailing-edge execution?