Why Closure Questions Are Common in Frontend Interviews

Closure is one of the most common topics in frontend interviews. Usually, a small code snippet is shown and candidates are asked to predict the output. At first glance, these questions seem simple, but they actually measure how well you understand JavaScript behavior.

Since JavaScript is behavior-driven, closures are not just theoretical concepts. They appear frequently in scopes, async operations, event handlers, and React hooks. Because of this, closure questions are very effective for evaluating real JavaScript knowledge.

What is Closure?

A closure is when a function remembers and accesses variables from its outer scope even after the outer function has finished executing.

Let's look at a simple example:

function outer() {
  let count = 0;

  function inner() {
    count++;
    console.log(count);
  }

  return inner;
}

const counter = outer();

counter(); // 1
counter(); // 2
counter(); // 3

Here, the inner function continues to access the count variable even after outer has executed. This is closure behavior.

Closures are a fundamental part of JavaScript and power many modern JavaScript patterns.

Why Closure Questions Are Popular in Interviews

Closure questions often use small but tricky examples. These examples measure multiple JavaScript concepts at once.

For example:

for (var i = 0; i < 3; i++) {
  setTimeout(() => {
    console.log(i);
  }, 100);
}

Many expect:

0
1
2

But the actual output is:

3
3
3

This happens because of closure and var scope behavior.

If we replace var with let:

for (let i = 0; i < 3; i++) {
  setTimeout(() => {
    console.log(i);
  }, 100);
}

Output becomes:

0
1
2

This example tests:

• Scope knowledge

• var vs let

• Closure behavior

• Async JavaScript

That's why closure questions are frequently used in interviews.

Closures in Real Projects

Closures are also used to generate functions:

function createMultiplier(multiplier) {
  return function(number) {
    return number * multiplier;
  };
}

Closures also appear in:

• Event handlers

• Callbacks

• Async operations

• React hooks

Example in React:

useEffect(() => {
  console.log(count);
}, []);

This is a classic stale closure example.

Closures may cause:

• Unexpected bugs

• Stale state values

• Hard-to-debug issues

What Closure Questions Measure

Closure questions typically measure:

• JavaScript behavior understanding

• Scope knowledge

• Async reasoning

• Debugging ability

This makes closure questions small but powerful signals.

Conclusion

Closures are one of JavaScript's core concepts. Scope, async behavior, and functional programming patterns rely heavily on closures.

That's why closure questions are common in frontend interviews. A small example can reveal deep JavaScript knowledge.

Closures are not just interview topics. Understanding closures helps developers write more predictable and stable code, especially in React applications.