uTools-Manuals/docs/javascript/Reference/Global_Objects/Array/ReduceRight.html

<article id="wikiArticle">
<div></div>
<div> </div>
<p><code><strong>reduceRight()</strong></code> 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。</p>
<div><iframe class="interactive interactive-js" frameborder="0" height="250" src="https://interactive-examples.mdn.mozilla.net/pages/js/array-reduce-right.html" width="100%"></iframe></div>
<p class="hidden">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 <a class="external" href="https://github.com/mdn/interactive-examples" rel="noopener">https://github.com/mdn/interactive-examples</a> and send us a pull request.</p>
<p>对于从左至右遍历的相似方法请参阅 <a href="Reference/Global_Objects/Array/reduce" title="reduce() 方法对数组中的每个元素执行一个由您提供的reducer函数(升序执行)，将其结果汇总为单个返回值。"><code>Array.prototype.reduce()</code></a>.</p>
<h2 id="语法">语法</h2>
<pre><code class="language-javascript"><code><em>arr</em>.reduceRight(<em>callback</em>[, <em>initialValue</em>])</code></code></pre>
<h3 id="参数">参数</h3>
<dl>
<dt><code>callback</code></dt>
<dd>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：
 <dl>
<dt><code>previousValue</code></dt>
<dd>上一次调用回调的返回值，或提供的 <code>initialValue</code></dd>
<dt><code>currentValue</code></dt>
<dd>当前被处理的元素</dd>
<dt><code>index</code></dt>
<dd>当前处理元素的索引</dd>
<dt><code>array</code></dt>
<dd>调用 <code>reduce</code> 的数组</dd>
</dl>
</dd>
<dt><code>initialValue</code></dt>
<dd>可作为第一次调用回调 <code>callback</code> 的第一个参数</dd>
<dt>
<h3 id="返回值">返回值</h3>
</dt>
<dd>
<p>执行之后的返回值</p>
</dd>
</dl>
<h2 id="描述">描述</h2>
<p><code>reduceRight</code> 为数组中每个元素调用一次 <code>callback</code> 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：初始值（或上次调用回调的返回值）、当前元素值、当前索引，以及调用 <code>reduce</code> 的数组。</p>
<p>可以像下面这样调用 <code>reduceRight</code> 的回调函数 <code>callback</code>：</p>
<pre><code  class="language-javascript">array.reduceRight(function(previousValue, currentValue, index, array) {
    // ...
});
</code></pre>
<p>首次调用回调函数时，<code>previousValue</code> 和 <code>currentValue</code> 可以是两个值之一。如果调用 <code>reduceRight</code> 时提供了 <code>initialValue</code> 参数，则 <code>previousValue</code> 等于 <code>initialValue</code>，<code>currentValue</code> 等于数组中的最后一个值。如果没有提供 <code>initialValue</code> 参数，则 <code>previousValue</code> 等于数组最后一个值， <code>currentValue</code> 等于数组中倒数第二个值。</p>
<p>如果数组为空，且没有提供 <code>initialValue</code> 参数，将会抛出一个 <code>TypeError 错误。如果数组只有一个元素且没有提供</code> <code>initialValue </code>参数，或者提供了 <code>initialValue</code> 参数，但是数组为空将会直接返回数组中的那一个元素或 <code>initialValue</code> 参数，而不会调用 <code>callback</code>。</p>
<p>该函数的完整执行过程见下例：</p>
<pre><code  class="language-javascript">[0, 1, 2, 3, 4].reduceRight(function(previousValue, currentValue, index, array) {
    return previousValue + currentValue;
});
</code></pre>
<p>回调将会被调用四次，每次调用的参数及返回值如下：</p>
<table style="width: 100%;">
<thead>
<tr>
<th scope="col"> </th>
<th scope="col"><code>previousValue</code></th>
<th scope="col"><code>currentValue</code></th>
<th scope="col"><code>index</code></th>
<th scope="col"><code>array</code></th>
<th scope="col">return value</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">first call</th>
<td>4</td>
<td>3</td>
<td>3</td>
<td><code>[0,1,2,3,4]</code></td>
<td><font face="Courier New, Andale Mono, monospace"><span style="line-height: normal;">7</span></font></td>
</tr>
<tr>
<th scope="row">second call</th>
<td>7</td>
<td><code>2</code></td>
<td><code>2</code></td>
<td><code>[0,1,2,3,4]</code></td>
<td><font face="Courier New, Andale Mono, monospace"><span style="line-height: normal;">9</span></font></td>
</tr>
<tr>
<th scope="row">third call</th>
<td>9</td>
<td>1</td>
<td>1</td>
<td><code>[0,1,2,3,4]</code></td>
<td><font face="Courier New, Andale Mono, monospace"><span style="line-height: normal;">10</span></font></td>
</tr>
<tr>
<th scope="row">fourth call</th>
<td>10</td>
<td>0</td>
<td>0</td>
<td><code>[0,1,2,3,4]</code></td>
<td><code>10</code></td>
</tr>
</tbody>
</table>
<p><code>reduceRight</code> 返回值是最后一次调用回调的返回值（<code>10）。</code></p>
<p>如果提供了一个 <code>initialValue</code> 参数，则结果如下：</p>
<pre><code  class="language-javascript">[0, 1, 2, 3, 4].reduceRight(function(previousValue, currentValue, index, array) {
    return previousValue + currentValue;
}, 10);
</code></pre>
<table style="width: 100%;">
<thead>
<tr>
<th scope="col"> </th>
<th scope="col"><code>previousValue</code></th>
<th scope="col"><code>currentValue</code></th>
<th scope="col"><code>index</code></th>
<th scope="col"><code>array</code></th>
<th scope="col">return value</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">first call</th>
<td><code>10</code></td>
<td>4</td>
<td>4</td>
<td><code>[0,1,2,3,4]</code></td>
<td><code>14</code></td>
</tr>
<tr>
<th scope="row">second call</th>
<td>14</td>
<td>3</td>
<td>3</td>
<td><code>[0,1,2,3,4]</code></td>
<td><code>17</code></td>
</tr>
<tr>
<th scope="row">third call</th>
<td>17</td>
<td><code>2</code></td>
<td><code>2</code></td>
<td><code>[0,1,2,3,4]</code></td>
<td><code>19</code></td>
</tr>
<tr>
<th scope="row">fourth call</th>
<td>19</td>
<td>1</td>
<td>1</td>
<td><code>[0,1,2,3,4]</code></td>
<td><font face="Courier New, Andale Mono, monospace"><span style="line-height: normal;">20</span></font></td>
</tr>
<tr>
<th scope="row">fifth call</th>
<td>20</td>
<td>0</td>
<td>0</td>
<td><code>[0,1,2,3,4]</code></td>
<td><code>20</code></td>
</tr>
</tbody>
</table>
<p><code>reduceRight</code> 返回值为 20。</p>
<h2 id="示例">示例</h2>
<h3 id="例子：求一个数组中所有值的和">例子：求一个数组中所有值的和</h3>
<pre><code  class="language-javascript">var total = [0, 1, 2, 3].reduceRight(function(a, b) {
    return a + b;
});
// total == 6
</code></pre>
<h3 id="例子：扁平化（flatten）一个元素为数组的数组">例子：扁平化（flatten）一个元素为数组的数组</h3>
<pre><code  class="language-javascript">var flattened = [[0, 1], [2, 3], [4, 5]].reduceRight(function(a, b) {
    return a.concat(b);
}, []);
// flattened is [4, 5, 2, 3, 0, 1]
</code></pre>
<h3 id="例子：reduce_与_reduceRight_之间的区别">例子：reduce 与 reduceRight 之间的区别</h3>
<pre><code  class="language-javascript">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"</code></pre>
<h2 id="兼容性旧环境（Polyfill）">兼容性旧环境（Polyfill）</h2>
<p><code>reduceRight</code> 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 <code>reduceRight</code> 的实现环境中使用它。</p>
<pre><code  class="language-javascript">// 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.reduce called on null or undefined');
    }
    if ('function' !== typeof callback) {
      throw new TypeError(callback + ' is not a function');
    }
    var t = Object(this), len = t.length &gt;&gt;&gt; 0, k = len - 1, value;
    if (arguments.length &gt;= 2) {
      value = arguments[1];
    } else {
      while (k &gt;= 0 &amp;&amp; !(k in t)) {
        k--;
      }
      if (k &lt; 0) {
        throw new TypeError('Reduce of empty array with no initial value');
      }
      value = t[k--];
    }
    for (; k &gt;= 0; k--) {
      if (k in t) {
        value = callback(value, t[k], k, t);
      }
    }
    return value;
  };
}
</code></pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.4.4.22" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">Array.prototype.reduceRight</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition.<br/>
    Implemented in JavaScript 1.8</td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-array.prototype.reduceright" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Array.prototype.reduceRight</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://tc39.github.io/ecma262/#sec-array.prototype.reduceright" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Array.prototype.reduceRight</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<div>
<div class="hidden">The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out <a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</div>
<p></p><div class="bc-data"><a class="bc-github-link external" href="https://github.com/mdn/browser-compat-data" rel="noopener">Update compatibility data on GitHub</a><table class="bc-table bc-table-js"><thead><tr class="bc-platforms"><td></td><th class="bc-platform-desktop" colspan="6"><span>Desktop</span></th><th class="bc-platform-mobile" colspan="7"><span>Mobile</span></th><th class="bc-platform-server" colspan="1"><span>Server</span></th></tr><tr class="bc-browsers"><td></td><th class="bc-browser-chrome"><span class="bc-head-txt-label bc-head-icon-chrome">Chrome</span></th><th class="bc-browser-edge"><span class="bc-head-txt-label bc-head-icon-edge">Edge</span></th><th class="bc-browser-firefox"><span class="bc-head-txt-label bc-head-icon-firefox">Firefox</span></th><th class="bc-browser-ie"><span class="bc-head-txt-label bc-head-icon-ie">Internet Explorer</span></th><th class="bc-browser-opera"><span class="bc-head-txt-label bc-head-icon-opera">Opera</span></th><th class="bc-browser-safari"><span class="bc-head-txt-label bc-head-icon-safari">Safari</span></th><th class="bc-browser-webview_android"><span class="bc-head-txt-label bc-head-icon-webview_android">Android webview</span></th><th class="bc-browser-chrome_android"><span class="bc-head-txt-label bc-head-icon-chrome_android">Chrome for Android</span></th><th class="bc-browser-edge_mobile"><span class="bc-head-txt-label bc-head-icon-edge_mobile">Edge Mobile</span></th><th class="bc-browser-firefox_android"><span class="bc-head-txt-label bc-head-icon-firefox_android">Firefox for Android</span></th><th class="bc-browser-opera_android"><span class="bc-head-txt-label bc-head-icon-opera_android">Opera for Android</span></th><th class="bc-browser-safari_ios"><span class="bc-head-txt-label bc-head-icon-safari_ios">Safari on iOS</span></th><th class="bc-browser-samsunginternet_android"><span class="bc-head-txt-label bc-head-icon-samsunginternet_android">Samsung Internet</span></th><th class="bc-browser-nodejs"><span class="bc-head-txt-label bc-head-icon-nodejs">Node.js</span></th></tr></thead><tbody><tr><th scope="row"><a href="Reference/Global_Objects/Array/reduceRight"><code>reduceRight</code></a></th><td class="bc-supports-yes bc-browser-chrome"><span class="bc-browser-name">Chrome</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-edge"><span class="bc-browser-name">Edge</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              12</td><td class="bc-supports-yes bc-browser-firefox"><span class="bc-browser-name">Firefox</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              3</td><td class="bc-supports-yes bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              9</td><td class="bc-supports-yes bc-browser-opera"><span class="bc-browser-name">Opera</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              10.5</td><td class="bc-supports-yes bc-browser-safari"><span class="bc-browser-name">Safari</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              4</td><td class="bc-supports-yes bc-browser-webview_android"><span class="bc-browser-name">WebView Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-chrome_android"><span class="bc-browser-name">Chrome Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-edge_mobile"><span class="bc-browser-name">Edge Mobile</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-firefox_android"><span class="bc-browser-name">Firefox Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              4</td><td class="bc-supports-yes bc-browser-opera_android"><span class="bc-browser-name">Opera Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-safari_ios"><span class="bc-browser-name">Safari iOS</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-samsunginternet_android"><span class="bc-browser-name">Samsung Internet Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-nodejs"><span class="bc-browser-name">nodejs</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td></tr></tbody></table><section class="bc-legend" id="sect1"><h3 class="offscreen" id="Legend">Legend</h3><dl><dt><span class="bc-supports-yes bc-supports">
<abbr class="bc-level bc-level-yes only-icon" title="Full support">
<span>Full support</span>
                  
                </abbr></span></dt><dd>Full support</dd></dl></section></div><p></p>
</div>
<h2 id="See_also" name="See_also">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/Array/reduce" title="reduce() 方法对数组中的每个元素执行一个由您提供的reducer函数(升序执行)，将其结果汇总为单个返回值。"><code>Array.prototype.reduce()</code></a></li>
</ul>
</article>