JavaScript Core ⚡ Recently updatedUpdated on Jun 30, 2026 — this content may still be evolving.

JavaScript Core: Mock Interview (45 Minutes)

Use with a friend. One interviewer, one candidate. Switch roles after.

Rules:

• Candidate answers out loud — no reading notes
• Interviewer asks 2+ follow-ups per question
• Section 4 requires writing code on paper or editor

Section 1: Warm-up (5 min)

1. “Tell me about a project where JavaScript was central.”
2. “What was the hardest bug you fixed in JS?”
3. “Which JS concept took you longest to understand?”

Interviewer: Note if they mention async, closures, or debugging process.

Section 2: Fundamentals (10 min)

1. “Explain the difference between var, let, and const.”
   • Follow-up: “What is the temporal dead zone?”
2. “What is a closure? Give a real use case.”
   • Follow-up: “Why does var in a loop with setTimeout print 3 three times?”
3. “Explain == vs === with an example that behaves differently.”

Section 3: this & Prototypes (8 min)

1. “How does this work in JavaScript?”
   • Follow-up: “What’s different about arrow functions and this?”
2. “What is the prototype chain?”
   • Follow-up: “How does instanceof work?”
3. “What do call, apply, and bind do?”

Section 4: Live Coding (12 min)

Pick ONE:

Option A: Implement debounce

“Write a debounce function. Explain when you’d use it.”

Evaluation: Correct timer logic, mentions search/resize use case.

Option B: Flatten array

“Flatten a nested array one level deep without .flat().”

function flatten(arr) {
  return arr.reduce((acc, val) =>
    acc.concat(Array.isArray(val) ? val : [val]), []);
}

Option C: Promise.all polyfill

“Implement a basic Promise.all.”

Follow-ups:

• “What happens if one promise rejects?”
• “Does order of results match input order?”

Section 5: Async & Event Loop (5 min)

1. Output question: “What prints and in what order?”

console.log('A');
setTimeout(() => console.log('B'), 0);
Promise.resolve().then(() => console.log('C'));
console.log('D');

Answer: A, D, C, B

2. “Explain the difference between microtasks and macrotasks.”
   • Follow-up: “Where does async/await fit in?”

Section 6: Tricky & Project (5 min)

Pick 2:

1. “What is 0.1 + 0.2 === 0.3? Why?”
2. “Shallow vs deep copy — how do you do each?”
3. “Walk me through how fetch works in your project.”
4. “What would you use: debounce or throttle for infinite scroll?”

Project connection:

• “Show me one async flow in your codebase.”
• “If you refactored one JS file, what would you improve?”

Evaluation Rubric

Debrief (5 min)

Candidate asks:

• Which topics felt shaky?
• Did I explain my code while writing?
• Was I honest when I didn’t know something?

Homework: Redo missed sections from interview-questions.md + one polyfill from polyfills.md blind.