Array.prototype.reduceRight()

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。

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,accumulatorcurrentValue 可以是两个值之一。如果调用 reduceRight 时提供了 initialValue 参数,则 accumulator等于 initialValuecurrentValue 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

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
reduceRightChrome Full support YesEdge Full support 12Firefox Full support 3IE Full support 9Opera Full support 10.5Safari Full support 4WebView Android Full support YesChrome Android Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes

Legend

Full support  
Full support

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