Closures in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a closure in JavaScript?

A closure is a function that remembers and can access variables from outside its own scope, even after the outer function has finished running.

beginner

Why do closures remember variables from their outer function?

Because when a function is created inside another function, it keeps a reference to the outer function's variables, so it can use them later.

intermediate

How can closures be useful in real life?

Closures help keep data private and create functions with their own memory, like counters or settings that remember past values.

intermediate

What will the following code output and why?<br><pre>function makeCounter() {<br>  let count = 0;<br>  return function() {<br>    count += 1;<br>    return count;<br>  }<br>}<br>const counter = makeCounter();<br>console.log(counter());<br>console.log(counter());</pre>

It will output:<br>1<br>2<br>Because the inner function remembers the variable 'count' from makeCounter and increases it each time it is called.

advanced

Can closures cause memory issues? Why or why not?

Yes, closures can keep variables in memory longer than needed if not managed carefully, because they hold references to outer variables even after the outer function ends.

What does a closure allow a function to do?

A Change the global variables directly

B Run faster than normal functions

C Access variables from its outer function even after the outer function has finished

D Create new variables in the global scope

Closures let a function remember and access variables from the outer function's scope even after that function has returned.

In this code, what will be logged?<br>

function outer() {<br>  let name = 'Alice';<br>  return function() {<br>    console.log(name);<br>  }<br>}<br>const inner = outer();<br>inner();

A null

B undefined

C Error

D 'Alice'

The inner function remembers the variable 'name' from the outer function and logs 'Alice'.

Which of these is NOT a benefit of closures?

A Automatically freeing memory

B Creating private variables

C Maintaining state between function calls

D Data privacy

Closures do not automatically free memory; in fact, they can keep variables in memory longer.

What happens if you create multiple closures from the same outer function?

A All closures share the same variables

B Each closure has its own separate copy of the outer variables

C Only the first closure works correctly

D Closures cannot be created multiple times

Each closure keeps its own copy of the outer function's variables, so they do not interfere with each other.

Which keyword is used to define a function that can create a closure?

A function

B var

C let

D const

The 'function' keyword defines functions, which can create closures when nested.

Explain in your own words what a closure is and why it is useful.

Think about how a function can keep some information even after it finishes.

Describe a simple example where a closure helps keep track of a count or state.

Imagine a counter that remembers how many times it was called.