Mock Interview Kits ⚡ Recently updatedUpdated on Jun 30, 2026 — this content may still be evolving.

JavaScript Mock Interview (45 Minutes)

Role: Core JavaScript round — appears in nearly every web dev interview, often as Round 1 or 2.

Interviewer: Stick to the timer. Push with “what happens if…?” follow-ups.
Candidate: Think out loud. For output questions, trace line by line before answering.

Pre-Interview Setup (Interviewer Only)

• Timer ready
• Live coding environment shared (or paper)
• Have 2 “tricky output” snippets ready (Section 4)
• Rubric: evaluation-rubric.md

Section 1: Warm-up (5 min)

Follow-up: “How did you finally understand it?”

Interviewer notes: Gauge confidence vs bluffing. Strong candidates name a specific concept (closures, async, this).

Section 2: Language Fundamentals (10 min)

Tricky output snippet #1 (read aloud, candidate traces):

console.log(1);
setTimeout(() => console.log(2), 0);
Promise.resolve().then(() => console.log(3));
console.log(4);
// Expected: 1, 4, 3, 2

Section 3: Data & Functions (8 min)

Stretch: “What is optional chaining (?.) and nullish coalescing (??)?”

Section 4: Tricky Output & Debugging (7 min)

Interviewer: Present one snippet at a time. Candidate must trace before answering.

Snippet A — Closure in loop:

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Expected: 3, 3, 3 — fix with let or IIFE

Follow-up: “How would you fix it?”

Snippet B — Type coercion:

console.log([] + []);
console.log([] + {});
console.log({} + []);
// Expected: "", "[object Object]", "[object Object]"

Interviewer question: “A bug report says if (user.count) hides valid zero values. How do you fix it?”

Expected: Use user.count != null or user.count !== undefined or nullish coalescing.

Section 5: Live Coding Task (10 min)

Interviewer reads aloud:

“Write a function debounce(fn, delay) that delays calling fn until delay ms have passed since the last call. Explain your approach first. You don’t need perfect syntax — I care about the logic.”

Timing:

Reference solution (interviewer only):

function debounce(fn, delay) {
  let timerId;
  return function (...args) {
    clearTimeout(timerId);
    timerId = setTimeout(() => fn.apply(this, args), delay);
  };
}

Follow-ups:

1. “What’s the difference between debounce and throttle?”
2. “Where would you use debounce in a real app?” (search input, resize handler)
3. “How would you add a cancel method?”
4. Alternate task if finished early: “Implement Array.prototype.myMap without using .map.”

Evaluation:

Section 6: DOM & Browser (3 min)

Pick 2 — fast fire:

Section 7: Project Connection (2 min)

Wrap-up (1 min)

Candidate asks one question. Interviewer ends professionally. Debrief separately.

Post-Interview Debrief

Score dimensions

• Language fundamentals (hoisting, closures, this, event loop)
• Async understanding
• Live coding (approach + correctness)
• Debugging mindset
• Communication

Homework by weak area

Retry: Run again in 5 days. Add a second live task: flatten nested array or groupBy.

Interviewer Notes

Strong signals: Traces code on paper before answering; mentions microtask vs macrotask unprompted; connects debounce to real UI.

Red flags: Guesses output without tracing; can’t explain own project’s API calls; says “I only use React, I don’t know vanilla JS.”