what-is-a-callback.md

What is a callback?

In short, a callback is a function that is passed as an argument to another function. Consider the example below.

var a = [1, 2, 3]

// console logs 1, 2, 3
// not this does not return anything
a.map(function(item) {console.log(item)})

Observe that we are passing the function (in this case an anonymous function) as the argument to map. This function is the callback.

Asynchronously

More importantly, callbacks are regularly used with in asynchronous functions. This is a typical practice to extract some result when we are waiting for some action to come back before proceeding. A simple example exists below.

var asyncExample = function(cb) {
  setTimeout(function() {
    cb()
  }, 5000)
}

var printSomething = function() {console.log('something')}

asyncExample(printSomething) // console logs 'something' after 5000ms

This is especially common when dealing with ajax calls or reading/writing actions.

Other notes

The context of a function can and will change when passed in as a callback. With that said, it means if a function or method references 'this' it will be necessary to use call/apply when trying to invoke that function in a callback.