map.js

Array.prototype.cmap = function(operation) {
  var output = [];
  for (var i = 0; i < this.length; i++) {
    output[i] = operation(this[i]);
  }
  return output;
}

var addTwo = function (a) {
  return a + 2;
}

var c = [1, 2, 3];
d = c.cmap(addTwo);
console.log(d);