callback.js

function foo(arg1, arg2, cb) {
  return arg1 + arg2 + cb(4)
}

console.log(foo(1, 2, function(num) {
  return num + 2
}))


var myArr = [1,2,3]

myArr.forEach(function(num, index, collection) {
  collection[index] = 'boo'
})

var myArr = [1,2,3]

myArr.forEach(function(num, index, collection) {
  return num + 2
})