Javascript Array reduce(callback)

// Array.prototype.reduce
Array.prototype.reduce = function reduce(callback) {
 if (typeof callback !== 'function') {
  throw new TypeError(callback + ' is not a function');
 }

 var array = this, length = array.length, index = 0, previousValue;

 while (index < length && !(index in array)) {
  ++index;/*from w  ww  .j  av  a  2s  .c  o m*/
 }

 previousValue = 2 in arguments ? arguments[2] : array[index];

 for (++index; index < length; ++index) {
  if (index in array) {
   previousValue = callback(previousValue, array[index], index, array);
  }
 }

 return previousValue;
};