32. 能不能实现数组reduce方法 ?

var result = ArraySpeciesCreate(array, length);

for (var i = 0; i < length; i++) { if (i in array) {

var element = array[i];

%CreateDataProperty(result, i, %_Call(f, receiver, element, i, array));

} }

return result;

}

1. 能不能实现数组reduce方法 ?

依照 ecma262 草案，实现的reduce的规范如下:

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。

} // 处理回调类型异常

if (Object.prototype.toString.call(callbackfn) != "[object Function]") { throw new TypeError(callbackfn + ' is not a function')

} let O = Object(this);

let len = O.length >>> 0;

let k = 0;

let accumulator = initialValue;

if (accumulator === undefined) { for(; k < len ; k++) {

// 查找原型链 if (k in O) {

accumulator = O[k];

k++;

break;

} }

} // 表示数组全为空

if(k === len && accumulator === undefined)

throw new Error('Each element of the array is empty');

for(;k < len; k++) { if (k in O) {

// 注意，核心！

accumulator = callbackfn.call(undefined, accumulator, O[k], k, O);

} }

return accumulator;

}

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:

function ArrayReduce(callback, current) { CHECK_OBJECT_COERCIBLE(this, "Array.prototype.reduce");

// Pull out the length so that modifications to the length in the // loop will not affect the looping and side effects are visible.

var array = TO_OBJECT(this);

var length = TO_LENGTH(array.length);

return InnerArrayReduce(callback, current, array, length, arguments.length);

}

function InnerArrayReduce(callback, current, array, length, argumentsLength) { if (!IS_CALLABLE(callback)) {

throw %make_type_error(kCalledNonCallable, callback);

}