Javascript Closure Interview Question

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Javascript Closure Interview Question

Understanding Javascript Closures: an Interview Question

A closure in JavaScript is a feature that allows a function to access variables from its outer scope, even after the function has been returned. This is made possible because the inner function maintains a reference to the variables in the outer scope, even if the outer function has already finished executing. This is a commonly asked interview question in JavaScript because it tests the understanding of scope and how variables are accessed in different scenarios.

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1 - What is a closure in JavaScript?

A closure is a function within a function that has access to the parent function's variables. The inner function retains a reference to the variables and parameters of the parent function, even after the parent function has returned.

2) How do you create a closure?

A closure is created by defining a function inside another function and then returning the inner function.

3) Why are closures useful?

Closures are useful because they allow for data privacy, reduce namespace collisions, and can be used to implement currying or partial application.

4) Can you give an example of a closure?

Sure, here's an example of a function that returns another function which has access to the parent function's variable:

function outerFunction() {

  let count = 0;

  function innerFunction() {




  return innerFunction;


const closure = outerFunction();

closure(); // output: 1

closure(); // output: 2

5) How does the JavaScript engine handle closures?

When a function is declared, a closure is created. The closure contains the environment where the function was declared, including all of the variables in scope at the time of creation.

6) What is the scope chain in closures?

The scope chain refers to the process of following the chain of parent functions up until the global scope to find variables that are not defined locally.

7) How can you access variables from outer functions within a closure?

Variables from outer functions can be accessed by the inner function either by passing them as arguments when the inner function is invoked, or by using the closure's scope chain to access them.

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8) Can a closure have access to newly declared variables within the parent function?

No, a closure only has access to variables that were in scope at the time it was defined.

9) How can you prevent closures from accessing variables from the parent function?

By using an immediately invoked function expression (IIFE), variables within the parent function can be “hidden” from the closure.

10) What is data hiding in closures?

Data hiding is a result of encapsulating variables within a closure, preventing them from being accessed outside of the closure.

11) How are closures different from regular functions?

Closures have access to variables outside of their own scope, while regular functions only have access to variables within their own scope.

12) What is currying in JavaScript?

Currying is a technique used to transform a function with multiple arguments into a sequence of functions, each accepting a single argument.

13) How can closures be used to implement currying?

By using closures, a function can return a new function with a partially applied set of arguments, allowing for currying.

14) What is partial application in JavaScript?

Partial application is similar to currying, but instead of returning a function for each argument, the original function is partially evaluated and returned. This can also be achieved with closures.

15) How can I debug closures?

Using the console.log() method within the parent and inner functions can help you trace the values of variables and parameters as the closure is executed.

16) Can closures cause memory leaks?

If not implemented properly, closures can create a reference to the parent function's variables and prevent them from being garbage collected. This can potentially cause memory leaks.

17) How can you avoid memory leaks with closures?

In order to avoid memory leaks, make sure to properly manage and dispose of closures, especially in long running applications.

18) Are closures supported in all JavaScript engines?

Yes, closures are a fundamental feature of JavaScript and are supported by all modern JavaScript engines.

19) How can you use closures to create private variables?

By using closures, you can create variables that are not accessible outside of the inner function, thus achieving data privacy.

20) Can closures be used for event handling?

Yes, closures are commonly used in event handling in JavaScript, as they allow for callback functions to have access to the variables of the parent function.


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