each-map.js

function each(array, callback) {
  var i, length = array.length;
  for (i = 0; i < length; i += 1) {
    callback(array[i]);
  }
}

function map(array, callback) {
  var newArray = [];
  each(array, function (item) {
    newArray.push(callback(item));
  });
  
  return newArray;
}

// Example
var a = [1, 2, 3];
var newA = map(a, function (item) {
  return item * 2;
});
console.log(newA);