polyfill.io? meh

_polyfill?.js

// https://github.com/jonathantneal/polyfill/blob/master/source/Array.prototype.reduce.js
// 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);
	}

	return previousValue;
};

no, that's a polyfill.js

"use strict"
// http://es5.github.io/#x15.4.4.21

var FUNCTION_CLASS = "[object Function]"
  , STRING_CLASS = "[object String]"
  , supportsStringAccess = "x"[0] == "x"
  , _toString = {}.toString
  , toObject = function(value){
      if(value == null) {
        throw new TypeError("'" + String(value) + "' is not an object")
      }
      if(!supportsStringAccess && value && _toString.call(value) == STRING_CLASS) {
        return value.split("")
      }
      return new Object(value)
    }

Array.prototype.reduce = function(fn){
  var array = toObject(this)
    , index = -1
    , length = array.length >>> 0
    , accummulator
    , hasInitialValue
    , reduced
    , item

  if(_toString.call(fn) != FUNCTION_CLASS) {
    throw new TypeError()
  }

  if(arguments.length > 1) {
    accummulator = arguments[1]
  } else {
    hasInitialValue = false
    while(++index < length) {
      if(!(hasInitialValue = index in array)) {
        continue
      }
      accummulator = array[index]
      break
    }
    if(!hasInitialValue) {
      throw new TypeError()
    }
  }

  while(++index < length) {
    if(!(index in array)) continue
    accummulator = fn.call(void 0, accummulator, array[index], index, array)
  }

  return accummulator
}