reduce.js

/*
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce
*/
// Production steps of ECMA-262, Edition 5, 15.4.4.21
// Reference: http://es5.github.io/#x15.4.4.21
if (!Array.prototype.reduce) {
  Array.prototype.reduce = function(callback, initialValue) {

    if (this === void 0 || this === null) {
      throw new TypeError('Array.prototype.reduce called on null or undefined');
    }

    if (callback.__class__ !== 'Function') {
      throw new TypeError(callback + ' is not a function');
    }

    var t = Object(this), len = t.length >>> 0, k = 0, value;

    if (arguments.length > 1) 
      {
        value = initialValue;
      } 
    else 
      {
        while (k < len && !(k in t)) {
          k++; 
        }
        if (k >= len) {
          throw new TypeError('Reduce of empty array with no initial value');
        }
        value = t[k++];
      }

    for (; k < len; k++) {
      if (k in t) {
        value = callback(value, t[k], k, t);
      }
    }
    return value;
  };
}