Why are js closures called closures?

31 July 2015

Why are js closures called closures?

I find it much easier to remember definitions of concepts and terms if I can somehow link it to an explanation or story behind its name. My naming stories may not always be historically accurate or especially exciting but they work for me so maybe you’ll find them useful too. If you’re here, I’m assuming that you already understand the concept but you’re looking for a way to remember/explain/imagine why it’s been named.

Why are js closures called closures?

A quick “define closure” google reveals the following:

noun 1. an act or process of closing something, especially an institution, thoroughfare, or frontier, or of being closed.

In javascript, a closure describes a function that refers to variables that would otherwise be garbage-collected/killed/closed down if that function didn’t exist. I.e. “If it weren’t for me you’d be dead by now kiddo” says the function to the fugitive variable whose being maintained outside the scope of its definition.

You can imagine that a closure is a function that’s promising to close the scope of these ‘deathrow variables’ as soon the function itself closes.

E.g. without closure:

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function Batman () {
  var privateVariable = "knickers" // <== deathrow variable
} // <== the executioner scope-closing curly bracket

var ben = new Batman()
return ben.privateVariable // => undefined
// because privateVariable was 'closed' by the closing curly bracket on line 3.

with closure:

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function Batman () {
  var privateVariable = "knickers"

  // here's the closure function
  this.getPreference = function() {
    return privateVariable
  }
}

var ben = new Batman()
return Ben.getPreference() // => "knickers"

Make sense? Lemme know below.