Javascript Array reduce(callback, initialValue)

// Array.prototype.reduce
Array.prototype.reduce = function reduce(callback, initialValue) {
  var array = this, previousValue = initialValue || 0;

  for (var index = 0, length = array.length; index < length; ++index) {
    previousValue = callback.call(window, previousValue, array[index], index, array);
  }// www  .  j a v a 2  s  .co m

  return previousValue;
};