JavaScript Interview Questions to Ask Before You Hire

12 JavaScript interview questions — with what to listen for

1. 1

   Explain the event loop: what happens when a timer, a promise, and a click handler are all pending?

   What a strong answer covers

   The call stack, task (macrotask) queue, and microtask queue — promises drain before the next task, timers are minimum delays not guarantees, and long synchronous work blocks everything. A candidate who can predict ordering in a small example owns the model; one who recites 'JavaScript is single-threaded' without consequences does not.

2. 2

   What is a closure, and give a real case where you used one deliberately — plus one where a closure caused a bug.

   What a strong answer covers

   Functions capturing their lexical scope; deliberate uses like private state, memoization, or event-handler factories. The bug side is the differentiator: stale values captured in loops or hooks, or listeners holding references that leak memory. Both directions confirm working knowledge.

3. 3

   async/await versus promise chains versus callbacks — how do you handle errors correctly in each?

   What a strong answer covers

   try/catch around awaited code with awareness that a missing await silently detaches errors, .catch placement semantics in chains, and unhandled-rejection consequences. Strong candidates mention Promise.all versus allSettled for aggregate error behavior and knowing when parallelism is safe.

4. 4

   How does prototypal inheritance actually work, and how do classes relate to it?

   What a strong answer covers

   Objects delegating property lookup up the prototype chain; class syntax as sugar over the same mechanism. Practical grounding — method sharing, instanceof behavior, why patching prototypes of built-ins is dangerous — beats textbook diagrams.

5. 5

   Explain how this is determined. Walk through the cases: method call, standalone function, arrow function, and an extracted method passed as a callback.

   What a strong answer covers

   Call-site binding for regular functions, lexical this for arrows, undefined (or global) in loose standalone calls, and the classic lost-this bug when extracting methods — fixed via arrows, bind, or restructuring. The extracted-callback case is where real bugs live, so weight it.

6. 6

   A page's interaction freezes for two seconds occasionally. How do you find the cause and fix it?

   What a strong answer covers

   Performance profiling to find long tasks, suspects like huge synchronous loops, layout thrashing from interleaved reads and writes, oversized JSON parsing, or synchronous storage access. Fixes: chunking work, web workers, debouncing, virtualization. Tool-first methodology is the grade.

7. 7

   How do memory leaks happen in JavaScript applications, and how do you hunt one down?

   What a strong answer covers

   Forgotten event listeners and intervals, detached DOM retained by references, ever-growing caches and maps, closures pinning large structures. Hunting via heap snapshots and comparing allocations over interactions. WeakMap/WeakRef awareness for cache design is a bonus.

8. 8

   What do var, let, and const each do, and why does the difference still matter beyond style?

   What a strong answer covers

   Function versus block scoping, hoisting and the temporal dead zone, const as binding-immutability not value-immutability. The loop-variable capture problem is the practical consequence worth demanding — it connects scoping to real bugs.

9. 9

   How do modules work — ESM versus CommonJS — and where do the two still collide in real projects?

   What a strong answer covers

   Static imports enabling tree shaking versus dynamic require, live bindings, and interop pain: dual-package hazards, default-export mismatches, config files and older tooling forcing CommonJS. Anyone shipping libraries or maintaining build tooling has scars here.

10. 10

    Debounce versus throttle: explain the difference and implement one of them on a whiteboard or editor.

    What a strong answer covers

    Debounce delays until quiet; throttle guarantees a maximum rate. The implementation exposes closure and timer fluency in ten lines — watch for correct timer clearing, argument forwarding, and this handling. A trailing/leading-edge discussion is senior polish.

11. 11

    How would you make a data-heavy page feel fast — what loading, caching, and rendering strategies apply?

    What a strong answer covers

    Code splitting and lazy loading, caching layers (HTTP, service worker, in-memory), optimistic UI, list virtualization, skeletons over spinners, and measuring with Core Web Vitals. Prioritizing by measurement rather than applying everything blindly is the judgment component.

12. 12

    Where does TypeScript help most, and what does it not protect you from?

    What a strong answer covers

    Compile-time safety on contracts, refactoring confidence, editor leverage; no runtime protection — unvalidated API responses and any-typed escape hatches still bite, so runtime validation at boundaries (zod-style) complements it. Framing TypeScript as standard professional practice is expected in 2026.

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