what-is-a-closure.md

What is a closure?

First, make sure to understand functional scope. Second, overall there seems to be a lack of consistency in regards to the definition of a closure so please be mindful when having intense conversations on this topic.

A closure is a function that retains access to variables of the context it was created it event after said function call has returned.

Typically after a function has returned, variables that were created inside the context of that function are no longer accessible. Consider the following.

var awFunc = function(first) {
  var someFunc = function(second) {
    return (first + second)
  }

  return someFunc('some')
}

var someNewFunc = awFunc('awe') // returns awesome
someNewFunc() // Uncaught ReferenceError

// Let's try to access the variables first and second
first // Uncaught ReferenceError
second // Uncaught ReferenceError

However we can refactor a little and come up with the following code

var awFunc = function(first) {
  var someFunc = function(second) {
    return (first + second)
  }

  return someFunc // notice function is not invoked
}

var someMoreFunc = awFunc('awe') // At this point awFunc has finished running

// We can wait a long time right here if we want

someMoreFunc('somer') // returns awesomer
// I still have access to variables of awFunc event after it's finished running

So one obvious advantage is to allow for separation of concerns when writing code. This become extremely useful when considering asynchronous code.

Related topic includes currying.