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.

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

Fundamentals

Q1. What is a closure? Easy

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

Q2. What is lexical scope? Easy

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

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

Q5. Can closures cause memory leaks? Hard

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

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

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

Q8. Can a closure modify the captured variable? Easy

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

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

Q11. How does let fix the loop bug? Medium

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);
}

Q13. What is variable shadowing? Medium

Q14. What is the temporal dead zone? Hard

Q15. Do closures work with block scope? Medium

Q16. What does this snippet print and why? Medium

Practical Patterns

Q17. What is the module pattern? Medium

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

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

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

Q19. How does memoization use closures? Medium

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

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);
  };
}

Q21. How do closures enable data privacy? Medium

Q22. What is a function factory? Medium

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

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

Advanced

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

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

Q26. Can two closures share state? Hard

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

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.

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].

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.

Quick Revision Table

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

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.