Question 1

What are template literals?

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 interpolation and newline-preservation claims below were actually run in Node. ## 1. Why This Even Matters — A Story First Writing a paragraph by hand with a regular quoted string is like typing a letter where every single line break has to be manually spelled out

Question 2

How would you use Object.groupBy() to group an array of orders by status, and how does it compare to a manual reduce()?

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 claim below was actually run — both implementations were executed against the identical real orders array and their outputs directly compared. ## 1. Why This Even Matters — A Story First Sorting a pile of mail into labeled bins by recipient is a task every office

Question 3

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 4

How does Array.prototype.with() let you replace one array element immutably, and where would that pattern actually help?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript immutable-data-pattern 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 a printed photograph versus a photo-editing app with an undo hist

Question 5

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 6

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 7

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 8

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 9

What does Array.filter do?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript array-method interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real non-mutation proof, the real chaining, and the real hole-skipping demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Sifting flour through a sieve produces a genuinely NEW, separate

Question 10

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

JAVASCRIPTDECODED.

Transmission log

What are template literals?

How would you use Object.groupBy() to group an array of orders by status, and how does it compare to a manual reduce()?

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

How does Array.prototype.with() let you replace one array element immutably, and where would that pattern actually help?

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

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 does Array.filter do?

What is hoisting in JavaScript?

What are static methods and properties?

What are optional chaining and nullish coalescing?

What is the difference between includes and indexOf?

Describe the concept of 'scope' in JavaScript.

What does Object.hasOwn() do differently from Object.prototype.hasOwnProperty.call(), and why is it the recommended modern replacement?

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

What is the difference between preventDefault and stopPropagation?

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

What is the difference between null and undefined?

What is the difference between find and filter?

What is globalThis?

What is the difference between Object.is, ==, and ===?

What is short-circuit evaluation?

What are the quirks of the typeof operator?

What is event bubbling and capturing?

What do Array.some and Array.every do?

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

What is the difference between synchronous and asynchronous code?

Why would you use performance.now() instead of Date.now() to measure how long a function takes to run?

What is the difference between localStorage, sessionStorage, and cookies?

How does Array.prototype.toReversed() avoid the classic bug of accidentally mutating a shared array reference?

What do Array.prototype.at() and String.prototype.at() do differently from bracket indexing, and why were they added?

Why would you add { passive: true } to a scroll or touchstart listener, and what problem does it solve for scroll performance?

How does instanceof work?

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

What is the difference between map and forEach?

What is destructuring?

What do Object.keys, values, and entries return?

What is JSON and why is it commonly used?

What is the difference between == and ===?

How does `try...catch` work in JavaScript?

Debounce vs throttle — implement debounce

How do you implement infinite currying in JavaScript?

What is the difference between Object.freeze, seal, and preventExtensions?

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

How would you use the BroadcastChannel API to sync application state across multiple open browser tabs?

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

Explain 'this' keyword in JavaScript.

How does the fetch API work?

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

What is memoization?

How does WeakMap allow garbage collection of keys?

How would you write a Proxy 'set' trap that validates a value before allowing an assignment, and what happens if you return false?

Implement a robust Event Emitter class.

What does Object.create do?

How do you correctly check for NaN?

What are modules in JavaScript?

What is lexical scoping and the scope chain?

How do you implement a deepFreeze function recursively?

What is a Promise and what are its states?

Explain event delegation.

What is an IIFE?

How does Array.reduce work?

What is event delegation?

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

How do you handle asynchronous operations in JavaScript?

How do you implement a custom Promise.all polyfill?

What do flat and flatMap do?

How does the 'this' keyword work?

How would you remove an item from an array immutably using toSpliced(), and how does it compare to splice()?

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

Why is 0.1 + 0.2 not exactly 0.3?

What are the spread and rest operators?

How do Import Attributes (`with { type: "json" }`) work and why did they replace import assertions?

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

What is the difference between arrow and regular functions?

How would you combine multiple AbortSignals or add a timeout to a fetch request using AbortSignal.any() and AbortSignal.timeout()?

What are the basics of regular expressions in JavaScript?