Array.prototype.reduce.js

if (!Array.prototype.reduce)
{
  Array.prototype.reduce = function(fun /*, initial*/)
  {
    var len = this.length;
    if (typeof fun != "function")
      throw new TypeError();

    // no value to return if no initial value and an empty array
    if (len == 0 && arguments.length == 1)
      throw new TypeError();

    var i = 0;
    if (arguments.length >= 2)
    {
      var rv = arguments[1];
    }
    else
    {
      do
      {
        if (i in this)
        {
          rv = this[i++];
          break;
        }

        // if array contains no values, no initial value to return
        if (++i >= len)
          throw new TypeError();
      }
      while (true);
    }

    for (; i < len; i++)
    {
      if (i in this)
        rv = fun.call(null, rv, this[i], i, this);
    }

    return rv;
  };
  Array.reduce = function(self, fun /*, initial*/)
  {
    if (arguments.length >= 2)
    {
      return Array.prototype.reduce.call(self, fun, arguments[1]);
    }
    return Array.prototype.reduce.call(self, fun);
  };
}
if (!Array.prototype.reduceRight)
{
  Array.prototype.reduceRight = function(fun /*, initial*/)
  {
    var len = this.length;
    if (typeof fun != "function")
      throw new TypeError();

    // no value to return if no initial value, empty array
    if (len == 0 && arguments.length == 1)
      throw new TypeError();

    var i = len - 1;
    if (arguments.length >= 2)
    {
      var rv = arguments[1];
    }
    else
    {
      do
      {
        if (i in this)
        {
          rv = this[i--];
          break;
        }

        // if array contains no values, no initial value to return
        if (--i < 0)
          throw new TypeError();
      }
      while (true);
    }

    for (; i >= 0; i--)
    {
      if (i in this)
        rv = fun.call(null, rv, this[i], i, this);
    }

    return rv;
  };
  Array.reduceRight = function(self, fun /*, initial*/)
  {
    if (arguments.length >= 2)
    {
      return Array.prototype.reduceRight.call(self, fun, arguments[1]);
    }
    return Array.prototype.reduceRight.call(self, fun);
  };
}