Custom Array Map

myArrayMap.js

Array.prototype.xmap = myCustomArrayMap;

function myCustomArrayMap (action) {
  var result = [];
  for (var i = 0; i < this.length; i++) {
    var args = [this[i], i, this];
    result.push(action.apply(null, args));
  }
  return result;
}

var res = [4, 5, 6].xmap(x => x + 6);
console.log(res, 'map result')