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

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

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 5

What is the difference between null and undefined?

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 check below was actually run in Node. ## 1. Why This Even Matters — A Story First Think of a form field on a job application. A field that was never shown to the applicant at all is like `undefined` — nobody had the chance to fill it in. A field that was shown, a

Question 6

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 7

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 8

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 9

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

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 was actually run in Node — not recited from memory. ## 1. Why This Even Matters — A Story First Imagine two shipping labels: one says "5 kg" and the other says "5000 g." A loose inspector (==) converts both to the same unit and says they match. A

Question 10

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

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript error-handling 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 factory floor with a trip wire around one risky machine. If nothing go

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Easy

What are template literals?

What is hoisting in JavaScript?

Describe the concept of 'scope' in JavaScript.

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 null and undefined?

What is globalThis?

What is the difference between synchronous and asynchronous code?

What is JSON and why is it commonly used?

Medium

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?

Explain 'this' keyword in JavaScript.

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

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

How do you correctly check for NaN?

Hard

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

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

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

What is the Reflect API?

What are Symbols and what are they used for?

Explain how prototypal inheritance differs from classical inheritance.

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

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