a callback in javascript for dummies

callback.js

// callbacks != promises
function doSomeAction() {
  return true
}

function appleTree() {
  return doSomeAction()
}

appleTree()

// --------------------------------------------------------------------

// we can decouple `doSomeAction` from `appleTree` by using a callback.
// it will provide more control over the execution flow.

function doSomeAction() {
  return true;
}

// instead of calling `doSomeAction` from within `appleTree`, we can decouple
// by completely removing the call to `doSomeAction`, and instead take a
// parameter (which will be a function [our callback]) that will be executed
// instead.
function appleTree(cb) {
  if (typeof cb === 'function') return cb() // make sure it's a function
  return false // no function? don't bother executing anything then. there is no callback to call.
}

appleTree(doSomeAction) // i can pass an arbitrary function method now. it's not dependent on `doSomeAction`
// or
appleTree(function() { // anonymous function style
  doSomeAction()
})
// or
appleTree.call(null, doSomeAction) // or using `call`

// --------------------------------------------------------------------

// a potential bottleneck of callbacks, dubbed 'callback hell,' refers
// to relying heavily on nested callbacks. it is in this case that it may
// make more sense to use an alternative form of async control; this may
// include concepts like event buses (event emitters), signals and as
// mentioned briefly, promise-based callbacks (promises).