Short guide to iteration functions!

iteration.js

var arr = [1,2,3,4]

// Array#map is for going from array A, to array-of-same-length B, with no side-effects.
var arrPrime = arr.map(function (num) { return num + 1 })

// forEach doesn't return anything useful (just `undefined`), so we use it for "side-effects".
var otherArr = []
arr.forEach(function (num) {
  if (num % 2 !== 0) {
    otherArr.push(num) // this is a side-effect
  } else {
    console.log(num) // this is also a side-effect
  }
})

// for loops are good for iteration gymnastics, or when there's no array to start from!
var finalArr = []
for (var i = 0; i < 10; i++) {
  if (i % 2 === 0) {
    i++
  }
  finalArr.shift(i)
}