map.js

// f is the function - it takes a single element of the array and returns a new element
function map(f, array) {
  var newArray = []
  for(var i = 0; i < array.length; ++i) {
    var oldItem = array[i]
    var newItem = f(oldItem)
    newArray.push(newItem)
  }
  return newArray
}