JavaScript Closures

If you’ve ever written a function in JavaScript, you already made use of closures.

It’s a key topic to understand, which has implications on the things you can do.

When a function is run, it’s executed with the scope that was in place when it was defined, and not with the state that’s in place when it is executed.

The scope basically is the set of variables which are visible.

A function remembers its Lexical Scope, and it’s able to access variables that were defined in the parent scope.

In short, a function has an entire baggage of variables it can access.

Let me immediately give an example to clarify this.

const bark = dog => {
  const say = `${dog} barked!`
  ;(() => console.log(say))()
}

bark(`Roger`)

This logs to the console Roger barked!, as expected.

What if you want to return the action instead:

const prepareBark = dog => {
  const say = `${dog} barked!`
  return () => console.log(say)
}

const bark = prepareBark(`Roger`)

bark()

This snippet also logs to the console Roger barked!.

Let’s make one last example, which reuses prepareBark for two different dogs:

const prepareBark = dog => {
  const say = `${dog} barked!`
  return () => {
    console.log(say)
  }
}

const rogerBark = prepareBark(`Roger`)
const sydBark = prepareBark(`Syd`)

rogerBark()
sydBark()

This prints

Roger barked!
Syd barked!

As you can see, the state of the variable say is linked to the function that’s returned from prepareBark().

Also notice that we redefine a new say variable the second time we call prepareBark(), but that does not affect the state of the first prepareBark() scope.

This is how a closure works: the function that’s returned keeps the original state in its scope.