Technical interview guide

JavaScript Fundamentals

Values and references, the four equality algorithms, scope and closures, this and arrow functions, prototypes and classes, the event loop, and modules.

Read: 46 min
Practice MCQs: 25
Interview QA: 25
Edition: v2
Editorial status: Reviewed
Relevant for: Frontend Engineer

Scope: ECMAScript 2024 and later, with Node behaviour cited from the active Node.js LTS line and browser behaviour from the WHATWG HTML event loop.

Interview QA

Treat each question like a live interview question: answer out loud first (structure, assumptions, tradeoffs), then open the model answer to spot gaps and rehearse a tighter follow-up.

Curated: 2026-10-05 · Written: 2026-10-05 · Reviewed: 2026-10-04

QA-1

Explain values against references and how it changes a production decision.

QA-2

How would you reason about numbers as IEEE-754 doubles in a system you own?

QA-3

Walk through strings as UTF-16 code units, including where engineers most often get it wrong.

QA-4

What does null against undefined guarantee, and what does it deliberately leave open?

QA-5

Describe the falsy set and truthiness and the evidence you would collect before relying on it.

QA-6

A teammate proposes a design that hinges on loose equality against strict equality. How do you evaluate it?

QA-7

Where does the NaN and negative-zero cases matter, and where is it irrelevant?

QA-8

Teach operator coercion and object-to-primitive conversion to an engineer who has only seen it as a rule of thumb.

QA-9

How do var, let, and const differ in terms of scoping, redeclaration, and the temporal dead zone (TDZ)?

QA-10

How does hoisting differ between function declarations, function expressions, and variable declarations during the creation phase of an execution context?

QA-11

How do closures capture variables by reference rather than by value, and how does this affect loops containing asynchronous callbacks?

QA-12

How does closures and retained memory change if the workload grows by two orders of magnitude?

QA-13

How is the value of 'this' determined at execution time in standard JavaScript functions, and how does it differ across strict and non-strict modes?

QA-14

In what scenarios should arrow functions be avoided due to their lexical binding of this?

QA-15

How would you explain call, apply, and bind without using the usual slogan?

QA-16

What failure would you inject to check a team's understanding of the prototype chain?

QA-17

How should on-call treat an alert that names classes over prototypes as the cause?

QA-18

What belongs in a runbook section on property descriptors and enumerability, and what does not?

QA-19

How would you review a pull request whose risk is shallow copies, deep copies, and structuredClone?

QA-20

What trade-off does arrays as objects with a length force that a junior answer usually skips?

QA-21

How would you brief product on why iteration protocols delays a ship date?

QA-22

What is the smallest experiment that would change your mind about the event loop and the microtask queue?

QA-23

How does promises and error propagation interact with rollback, and where do people ignore that?

QA-24

What would you ask a candidate who recites await, sequential or concurrent but cannot apply it?

QA-25