JavaScript Closures Interview Questions 2026: 26 Q&A

Last Updated: June 2026 | Level: Freshers to Mid-Level | Read Time: ~15 min

Closures are the concept that most distinguishes confident JavaScript developers in interviews. Candidates report that the definition, the loop closure bug, and practical uses like the module pattern and currying are guaranteed questions. This guide covers 26 closure questions with full answers, runnable code, and predict-the-output traps. Behaviour reflects the official MDN documentation; confirm any scope detail on MDN.

Pair this with JavaScript Interview Questions for Freshers 2026 and JavaScript Output Questions 2026.

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Table of Contents

1. Fundamentals (Q1 to Q9)
2. The Loop Bug and Scope (Q10 to Q16)
3. Practical Patterns (Q17 to Q23)
4. Advanced (Q24 to Q26)
5. Predict the Output
6. Quick Revision Table
7. Frequently Asked Questions

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Fundamentals

Q1. What is a closure? Easy

function outer() {
  let count = 0;
  return function() { return ++count; };
}
const inc = outer();
inc(); inc();   // 1, then 2

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Q2. What is lexical scope? Easy

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Q3. How does a closure keep a variable alive? Medium

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Q4. What is the difference between scope and closure? Medium

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Q5. Can closures cause memory leaks? Hard

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Q6. Do closures capture by value or by reference? Hard

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Q7. What is an IIFE and how does it relate to closures? Medium

(function() { /* private scope */ })();

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Q8. Can a closure modify the captured variable? Easy

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Q9. How many closures are created in a function with two inner functions? Hard

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The Loop Bug and Scope

Q10. What is the classic closure-in-a-loop bug? Medium

for (var i = 0; i < 3; i++) setTimeout(() => console.log(i)); // 3 3 3
for (let i = 0; i < 3; i++) setTimeout(() => console.log(i)); // 0 1 2

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Q11. How does let fix the loop bug? Medium

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Q12. How would you fix the loop bug without let? Hard

for (var i = 0; i < 3; i++) {
  (function(j) { setTimeout(() => console.log(j)); })(i);
}

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Q13. What is variable shadowing? Medium

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Q14. What is the temporal dead zone? Hard

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Q15. Do closures work with block scope? Medium

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Q16. What does this snippet print and why? Medium

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Practical Patterns

Q17. What is the module pattern? Medium

const counter = (function() {
  let c = 0;
  return { inc: () => ++c, get: () => c };
})();

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Q18. What is currying and how does it use closures? Medium

const add = a => b => a + b;
add(2)(3);   // 5

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Q19. How does memoization use closures? Medium

function memoize(fn) {
  const cache = {};
  return x => x in cache ? cache[x] : (cache[x] = fn(x));
}

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Q20. How do you implement a debounce with closures? Hard

function debounce(fn, ms) {
  let t;
  return (...args) => {
    clearTimeout(t);
    t = setTimeout(() => fn(...args), ms);
  };
}

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Q21. How do closures enable data privacy? Medium

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Q22. What is a function factory? Medium

const triple = (n => x => x * n)(3);
triple(5);   // 15

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Q23. How do once-only functions use closures? Medium

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Advanced

Q24. What is the difference between a closure and a regular function? Medium

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Q25. How do closures interact with the garbage collector? Hard

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Q26. Can two closures share state? Hard

function pair() {
  let v = 0;
  return [() => ++v, () => v];
}
const [set, get] = pair();
set(); get();   // 1

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Predict the Output

Snippet 1

function make() {
  let x = 0;
  return { inc: () => ++x, val: () => x };
}
const m = make();
m.inc(); m.inc();
console.log(m.val());

Output:

2

Why: Both methods close over the same x, so two increments leave it at 2.

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Snippet 2

const fns = [];
for (var i = 0; i < 3; i++) fns.push(() => i);
console.log(fns.map(f => f()));

Output:

[ 3, 3, 3 ]

Why: var shares one binding; all closures read the final i of 3. Use let for [0, 1, 2].

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Snippet 3

const add = a => b => a + b;
const add5 = add(5);
console.log(add5(3), add5(10));

Output:

8 15

Why: add5 is a closure capturing a=5; each call supplies b.

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Quick Revision Table

Concept	Key takeaway
Closure	function plus captured lexical scope
Capture	by reference, not snapshot
Loop bug	var shares, let per-iteration
Module pattern	IIFE returns public API, hides state
Currying	chained single-arg closures
Memoization	private cache in closure
Leak risk	captured big object stays alive

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Frequently Asked Questions

What is a closure in JavaScript in simple terms?

A closure is a function bundled with references to its surrounding lexical scope, so it can read and update outer variables even after the outer function has returned. Candidates report this is the single most asked JavaScript concept.

Why is the loop closure bug asked so often?

Because it tests whether you understand that var closures share one binding while let closures get a fresh binding per iteration. It is the most common closure trap in interviews.

What are practical uses of closures?

Data privacy with the module pattern, function factories and currying, memoization caches, debounce and throttle helpers, and event handlers that remember state. Interviewers expect at least two real uses.

Do closures capture the value or the variable?

The variable, by reference, not a snapshot of its value. That is exactly why a var-loop closure sees the final value while a let-loop closure sees its own iteration value.

Can closures cause memory leaks?

Yes. A reachable closure keeps its captured variables (and anything they reference) alive. Detach handlers and clear references in long-lived applications to let the garbage collector reclaim them.

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Related Articles

• JavaScript Interview Questions for Freshers 2026, the master set
• JavaScript Output Questions 2026, tricky snippets
• JavaScript Async Await Interview Questions 2026, promises and the event loop
• JavaScript Coding Questions With Solutions 2026, solved problems

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Confirm any scope or closure detail on the official MDN documentation before your interview. This guide reflects candidate-reported patterns and public preparation resources as of June 2026.