JavaScript Closures Simply Explained

They key to understanding closures is scope, because closure is nothing more than the outer scope of a function.

Let's look at an example:

/* Outer scope */
let someVariable = 'someValue'

function printVariable() {
  /* Inner scope */
  console.log(someVariable)
}

printVariable() // someValue

The function printVariable has access to any variables defined outside of it, e.g. someVariable. The successful log of this variable proves so.

Nested functions

Closures are often discussed in the context of nested functions, using the following typical example:

function outerFunction(outerVariable) {
  return function innerFunction(innerVariable) { 
    console.log(outerVariable, innerVariable)
  }
}

const anotherFunction = outerFunction('outside')
anotherFunction('inside') // outside, inside

As you can see, innerFunction still has access to any variables from its parent function, even after the parent has been called.

However, whenever you work with closures, you should be acutely aware ⚠️ of the concept of stale closures!