reduceRight()
方法接受一个函数作为累加器(accumulator)和数组的每个值(从右到左)将其减少为单个值。
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
对于从左至右遍历的相似方法请参阅 Array.prototype.reduce()
.
语法
arr.reduceRight(callback(accumulator, currentValue[, index[, array]])[, initialValue])
参数
-
callback
-
一个回调函数,用来操作数组中的每个元素,可接受四个参数:
-
accumulator
-
上一次调用回调的返回值,或提供的
initialValue。
-
currentValue
- 当前被处理的元素。
-
index
可选 - 数组中当前被处理的元素的索引。
-
array
可选 -
调用
reduceRight()
的数组
-
-
initialValue
可选 - 值用作回调的第一次调用的累加器。如果未提供初始值,则将使用并跳过数组中的最后一个元素。在没有初始值的空数组上调用reduce或reduceRight就会创建一个TypeError。
-
返回值
-
执行之后的返回值。
描述
reduceRight
为数组中每个元素调用一次 callback
回调函数,但是数组中被删除的索引或从未被赋值的索引会跳过。回调函数接受四个参数:the initial value初始值(或上次调用回调的返回值)、the value of the current element当前元素值、the current index当前索引,以及调用迭代的当前数组。
可以像下面这样调用 reduceRight
的回调函数 callback
:
array.reduceRight(function(accumulator, currentValue, index, array) {
// ...
});
首次调用回调函数时,accumulator
和 currentValue
可以是两个值之一。如果调用 reduceRight
时提供了 initialValue
参数,则 accumulator
等于 initialValue
,currentValue
等于数组中的最后一个值。如果没有提供 initialValue
参数,则 accumulator
等于数组最后一个值, currentValue
等于数组中倒数第二个值。
如果数组为空,且没有提供 initialValue
参数,将会抛出一个 TypeError 错误。如果数组只有一个元素且没有提供
initialValue
参数,或者提供了 initialValue
参数,但是数组为空将会直接返回数组中的那一个元素或 initialValue
参数,而不会调用 callback
。
该函数的完整执行过程见下例:
[0, 1, 2, 3, 4].reduceRight(function(previousValue, currentValue, index, array) { return previousValue + currentValue; });
回调将会被调用四次,每次调用的参数及返回值如下:
previousValue |
currentValue |
index |
array |
return value | |
---|---|---|---|---|---|
first call | 4 | 3 | 3 | [0,1,2,3,4] |
7 |
second call | 7 | 2 |
2 |
[0,1,2,3,4] |
9 |
third call | 9 | 1 | 1 | [0,1,2,3,4] |
10 |
fourth call | 10 | 0 | 0 | [0,1,2,3,4] |
10 |
reduceRight
返回值是最后一次调用回调的返回值(10)。
如果提供了一个 initialValue
参数,则结果如下:
[0, 1, 2, 3, 4].reduceRight(function(previousValue, currentValue, index, array) { return previousValue + currentValue; }, 10);
previousValue |
currentValue |
index |
array |
return value | |
---|---|---|---|---|---|
first call | 10 |
4 | 4 | [0,1,2,3,4] |
14 |
second call | 14 | 3 | 3 | [0,1,2,3,4] |
17 |
third call | 17 | 2 |
2 |
[0,1,2,3,4] |
19 |
fourth call | 19 | 1 | 1 | [0,1,2,3,4] |
20 |
fifth call | 20 | 0 | 0 | [0,1,2,3,4] |
20 |
reduceRight
返回值为 20。
示例
求一个数组中所有值的和
var sum = [0, 1, 2, 3].reduceRight(function(a, b) {
return a + b;
});
// sum is 6
扁平化(flatten)一个元素为数组的数组
var flattened = [[0, 1], [2, 3], [4, 5]].reduceRight(function(a, b) { return a.concat(b); }, []); // flattened is [4, 5, 2, 3, 0, 1]
运行一个带有回调每个函数将其结果传给下一个的异步函数列表
const waterfall = (...functions) => (callback, ...args) =>
functions.reduceRight(
(composition, fn) => (...results) => fn(composition, ...results),
callback
)(...args);
const randInt = max => Math.floor(Math.random() * max)
const add5 = (callback, x) => {
setTimeout(callback, randInt(1000), x + 5);
};
const mult3 = (callback, x) => {
setTimeout(callback, randInt(1000), x * 3);
};
const sub2 = (callback, x) => {
setTimeout(callback, randInt(1000), x - 2);
};
const split = (callback, x) => {
setTimeout(callback, randInt(1000), x, x);
};
const add = (callback, x, y) => {
setTimeout(callback, randInt(1000), x + y);
};
const div4 = (callback, x) => {
setTimeout(callback, randInt(1000), x / 4);
};
const computation = waterfall(add5, mult3, sub2, split, add, div4);
computation(console.log, 5) // -> 14
// same as:
const computation2 = (input, callback) => {
const f6 = x=> div4(callback, x);
const f5 = (x, y) => add(f6, x, y);
const f4 = x => split(f5, x);
const f3 = x => sub2(f4, x);
const f2 = x => mult3(f3, x);
add5(f2, input);
}
reduce 与 reduceRight 之间的区别
var a = ['1', '2', '3', '4', '5']; var left = a.reduce(function(prev, cur) { return prev + cur; }); var right = a.reduceRight(function(prev, cur) { return prev + cur; }); console.log(left); // "12345" console.log(right); // "54321"
定义可组合函数
组合函数的概念简单,它结合了n个函数。它是一个从右向左流动的函数,用上一个函数的输出调用每个函数。
/**
* Function Composition is way in which result of one function can
* be passed to another and so on.
*
* h(x) = f(g(x))
*
* Function execution happens right to left
*
* https://en.wikipedia.org/wiki/Function_composition
*/
const compose = (...args) => (value) => args.reduceRight((acc, fn) => fn(acc), value)
// Increament passed number
const inc = (n) => n + 1
// Doubles the passed value
const double = (n) => n * 2
// using composition function
console.log(compose(double, inc)(2)); // 6
// using composition function
console.log(compose(inc, double)(2)); // 5
兼容性旧环境(Polyfill)
reduceRight
被添加到 ECMA-262 标准第 5 版,因此它在某些实现环境中可能不被支持。把下面的代码添加到脚本开头可以解决此问题,从而允许在那些没有原生支持 reduceRight
的实现环境中使用它。
// Production steps of ECMA-262, Edition 5, 15.4.4.22 // Reference: http://es5.github.io/#x15.4.4.22 if ('function' !== typeof Array.prototype.reduceRight) { Array.prototype.reduceRight = function(callback /*, initialValue*/) { 'use strict'; if (null === this || 'undefined' === typeof this) { throw new TypeError('Array.prototype.reduceRight called on null or undefined'); } if ('function' !== typeof callback) { throw new TypeError(callback + ' is not a function'); } var t = Object(this), len = t.length >>> 0, k = len - 1, value; if (arguments.length >= 2) { value = arguments[1]; } else { while (k >= 0 && !(k in t)) { k--; } if (k < 0) { throw new TypeError('reduceRight of empty array with no initial value'); } value = t[k--]; } for (; k >= 0; k--) { if (k in t) { value = callback(value, t[k], k, t); } } return value; }; }
规范
Specification | Status | Comment |
---|---|---|
ECMAScript 5.1 (ECMA-262) Array.prototype.reduceRight |
Standard | Initial definition. Implemented in JavaScript 1.8 |
ECMAScript 2015 (6th Edition, ECMA-262) Array.prototype.reduceRight |
Standard | |
ECMAScript Latest Draft (ECMA-262) Array.prototype.reduceRight |
Draft |
浏览器兼容性
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
reduceRight |
Chrome Full support 3 | Edge Full support 12 | Firefox Full support 3 | IE Full support 9 | Opera Full support 10.5 | Safari Full support 4.1 | WebView Android Full support ≤37 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support Yes | Safari iOS Full support 4 | Samsung Internet Android Full support 1.0 | nodejs Full support Yes |
Legend
- Full support
- Full support