依照 ecma262 草案，实现的reduce的规范如下:

    reduce.jpg

    其中有几个核心要点:

    1、初始值不传怎么处理

    2、回调函数的参数有哪些，返回值如何处理。

    1. Array.prototype.reduce  = function(callbackfn, initialValue) {
    2.   // 异常处理，和 map 一样
    3.   // 处理数组类型异常
    4.   if (this === null || this === undefined) {
    5.     throw new TypeError("Cannot read property 'reduce' of null or undefined");
    6.   }
    7.   // 处理回调类型异常
    8.   if (Object.prototype.toString.call(callbackfn) != "[object Function]") {
    9.     throw new TypeError(callbackfn + ' is not a function')
    10.   }
    11.   let O = Object(this);
    12.   let len = O.length >>> 0;
    13.   let k = 0;
    14.   let accumulator = initialValue;
    15.   if (accumulator === undefined) {
    16.     for(; k < len ; k++) {
    17.       // 查找原型链
    18.       if (k in O) {
    19.         accumulator = O[k];
    20.         k++;
    21.         break;
    22.       }
    23.     }
    24.     // 循环结束还没退出，就表示数组全为空
    25.     throw new Error('Each element of the array is empty');
    26.   }
    27.   for(;k < len; k++) {
    28.     if (k in O) {
    29.       // 注意，核心！
    30.       accumulator = callbackfn.call(undefined, accumulator, O[k], O);
    31.     }
    32.   }
    33.   return accumulator;
    34. }

    其实是从最后一项开始遍历，通过原型链查找跳过空项。

    最后给大家奉上V8源码，以供大家检查:

    1. function ArrayReduce(callback, current) {
    2.   CHECK_OBJECT_COERCIBLE(this, "Array.prototype.reduce");
    4.   // Pull out the length so that modifications to the length in the
    5.   // loop will not affect the looping and side effects are visible.
    6.   var array = TO_OBJECT(this);
    7.   var length = TO_LENGTH(array.length);
    8.   return InnerArrayReduce(callback, current, array, length,
    9.                           arguments.length);
    10. }
    12. function InnerArrayReduce(callback, current, array, length, argumentsLength) {
    13.   if (!IS_CALLABLE(callback)) {
    14.     throw %make_type_error(kCalledNonCallable, callback);
    15.   }
    17.   var i = 0;
    18.   find_initial: if (argumentsLength < 2) {
    19.     for (; i < length; i++) {
    20.       if (i in array) {
    21.         current = array[i++];
    22.         break find_initial;
    23.       }
    24.     }
    25.     throw %make_type_error(kReduceNoInitial);
    26.   }
    28.   for (; i < length; i++) {
    29.     if (i in array) {
    30.       var element = array[i];
    31.       current = callback(current, element, i, array);
    32.     }
    33.   }
    34.   return current;
    35. }

    参考:

    V8源码

    ecma262草案