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+---
+title: Array.prototype.reduce()
+slug: Web/JavaScript/Reference/Global_Objects/Array/Reduce
+tags:
+  - Array
+  - ECMAScript 5
+  - JavaScript
+  - Method
+  - Prototype
+  - Reduce
+  - Reference
+translation_of: Web/JavaScript/Reference/Global_Objects/Array/Reduce
+---
+<p>{{JSRef}}</p>
+
+<p><code><strong>reduce()</strong></code> 方法对数组中的每个元素执行一个由您提供的<strong>reducer</strong>函数(升序执行)，将其结果汇总为单个返回值。</p>
+
+<div>{{EmbedInteractiveExample("pages/js/array-reduce.html")}}</div>
+
+<div class="hidden">
+<p>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 href="https://github.com/mdn/interactive-examples">https://github.com/mdn/interactive-examples</a> and send us a pull request. </p>
+</div>
+
+<div>
+<p><strong>reducer</strong> 函数接收4个参数:</p>
+
+<ol>
+ <li>Accumulator (acc) (累计器)</li>
+ <li>Current Value (cur) (当前值)</li>
+ <li>Current Index (idx) (当前索引)</li>
+ <li>Source Array (src) (源数组)</li>
+</ol>
+
+<p>您的 <strong>reducer</strong> 函数的返回值分配给累计器，该返回值在数组的每个迭代中被记住，并最后成为最终的单个结果值。</p>
+</div>
+
+<h2 id="语法">语法</h2>
+
+<pre class="notranslate"><var>arr.reduce(callback(accumulator, currentValue[, index[, array]])[, initialValue])</var></pre>
+
+<h3 id="参数">参数</h3>
+
+<dl>
+ <dt><code>callback</code></dt>
+ <dd>执行数组中每个值 (如果没有提供 <code>initialValue则第一个值除外</code>)的函数，包含四个参数：</dd>
+ <dd>
+ <dl>
+ </dl>
+ <strong><code>accumulator</code></strong>
+
+ <dl>
+  <dd>
+  <p>累计器累计回调的返回值; 它是上一次调用回调时返回的累积值，或<code>initialValue</code>（见于下方）。</p>
+  </dd>
+  <dt><code>currentValue</code></dt>
+  <dd>数组中正在处理的元素。</dd>
+  <dt><code>index</code> {{optional_inline}}</dt>
+  <dd>数组中正在处理的当前元素的索引。 如果提供了<code>initialValue</code>，则起始索引号为0，否则从索引1起始。</dd>
+  <dt><code>array</code>{{optional_inline}}</dt>
+  <dd>调用<code>reduce()</code>的数组</dd>
+ </dl>
+ </dd>
+ <dt><code>initialValue</code>{{optional_inline}}</dt>
+ <dd>作为第一次调用 <code>callback</code>函数时的第一个参数的值。 如果没有提供初始值，则将使用数组中的第一个元素。 在没有初始值的空数组上调用 reduce 将报错。</dd>
+</dl>
+
+<h3 id="返回值">返回值</h3>
+
+<p>函数累计处理的结果</p>
+
+<h2 id="描述">描述</h2>
+
+<p><code>reduce</code>为数组中的每一个元素依次执行<code>callback</code>函数，不包括数组中被删除或从未被赋值的元素，接受四个参数：</p>
+
+<ul>
+ <li><code>accumulator 累计器</code></li>
+ <li><code>currentValue 当前值</code></li>
+ <li><code>currentIndex 当前索引</code></li>
+ <li><code>array 数组</code></li>
+</ul>
+
+<p>回调函数第一次执行时，<code>accumulator</code><span style="line-height: 1.5;"> 和</span><code>currentValue</code>的取值有两种情况：如果调用<code>reduce()</code>时提供了<code>initialValue</code>，<code>accumulator</code>取值为<code>initialValue</code>，<code>currentValue</code>取数组中的第一个值；如果没有提供 <code>initialValue</code>，那么<code>accumulator</code>取数组中的第一个值，<code>currentValue</code>取数组中的第二个值。</p>
+
+<div class="note">
+<p><strong>注意：</strong>如果没有提供<code>initialValue</code>，reduce 会从索引1的地方开始执行 callback 方法，跳过第一个索引。如果提供<code>initialValue</code>，从索引0开始。</p>
+</div>
+
+<p>如果数组为空且没有提供<code>initialValue</code>，会抛出{{jsxref("TypeError")}} 。如果数组仅有一个元素（无论位置如何）并且没有提供<code>initialValue</code>， 或者有提供<code>initialValue</code>但是数组为空，那么此唯一值将被返回并且<code>callback</code>不会被执行。</p>
+
+<p>提供初始值通常更安全，正如下面的例子，如果没有提供<code>initialValue</code>，则可能有四种输出：</p>
+
+<pre class="brush: js notranslate">var maxCallback = ( acc, cur ) =&gt; Math.max( acc.x, cur.x );
+var maxCallback2 = ( max, cur ) =&gt; Math.max( max, cur );
+
+// reduce() 没有初始值
+[ { x: 2 }, { x: 22 }, { x: 42 } ].reduce( maxCallback ); // NaN
+[ { x: 2 }, { x: 22 }            ].reduce( maxCallback ); // 22
+[ { x: 2 }                       ].reduce( maxCallback ); // { x: 2 }
+[                                ].reduce( maxCallback ); // TypeError
+
+// map/reduce; 这是更好的方案，即使传入空数组或更大数组也可正常执行
+[ { x: 22 }, { x: 42 } ].map( el =&gt; el.x )
+                        .reduce( maxCallback2, -Infinity );
+</pre>
+
+<h3 id="reduce_如何运行">reduce() 如何运行</h3>
+
+<p>假如运行下段<code>reduce()</code>代码：</p>
+
+<pre class="brush:js notranslate">[0, 1, 2, 3, 4].reduce(function(accumulator, currentValue, currentIndex, array){
+  return accumulator + currentValue;
+});
+</pre>
+
+<p>callback 被调用四次，每次调用的参数和返回值如下表：</p>
+
+<table>
+ <thead>
+  <tr>
+   <th scope="col"><code>callback</code></th>
+   <th scope="col"><code>accumulator</code></th>
+   <th scope="col"><code>currentValue</code></th>
+   <th scope="col"><code>currentIndex</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>0</code></td>
+   <td><code>1</code></td>
+   <td><code>1</code></td>
+   <td><code>[0, 1, 2, 3, 4]</code></td>
+   <td><code>1</code></td>
+  </tr>
+  <tr>
+   <th scope="row">second call</th>
+   <td><code>1</code></td>
+   <td><code>2</code></td>
+   <td><code>2</code></td>
+   <td><code>[0, 1, 2, 3, 4]</code></td>
+   <td><code>3</code></td>
+  </tr>
+  <tr>
+   <th scope="row">third call</th>
+   <td><code>3</code></td>
+   <td><code>3</code></td>
+   <td><code>3</code></td>
+   <td><code>[0, 1, 2, 3, 4]</code></td>
+   <td><code>6</code></td>
+  </tr>
+  <tr>
+   <th scope="row">fourth call</th>
+   <td><code>6</code></td>
+   <td><code>4</code></td>
+   <td><code>4</code></td>
+   <td><code>[0, 1, 2, 3, 4]</code></td>
+   <td><code>10</code></td>
+  </tr>
+ </tbody>
+</table>
+
+<table style="width: 100%;">
+ <tbody>
+  <tr>
+  </tr>
+ </tbody>
+</table>
+
+<p>由<code>reduce</code>返回的值将是最后一次回调返回值（10）。</p>
+
+<p>你还可以使用{{jsxref("Functions/Arrow_functions", "箭头函数","",1)}}来代替完整的函数。 下面的代码将产生与上面的代码相同的输出：</p>
+
+<pre class="brush: js notranslate">[0, 1, 2, 3, 4].reduce((prev, curr) =&gt; prev + curr );</pre>
+
+<p>如果你打算提供一个初始值作为<code>reduce()</code>方法的第二个参数，以下是运行过程及结果：</p>
+
+<pre class="brush: js notranslate">[0, 1, 2, 3, 4].reduce((accumulator, currentValue, currentIndex, array) =&gt; {
+    return accumulator + currentValue
+}, 10)
+</pre>
+
+<table>
+ <thead>
+  <tr>
+   <th scope="col"><code>callback</code></th>
+   <th scope="col"><code>accumulator</code></th>
+   <th scope="col"><code>currentValue</code></th>
+   <th scope="col"><code>currentIndex</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><code>0</code></td>
+   <td><code>0</code></td>
+   <td><code>[0, 1, 2, 3, 4]</code></td>
+   <td><code>10</code></td>
+  </tr>
+  <tr>
+   <th scope="row">second call</th>
+   <td><code>10</code></td>
+   <td><code>1</code></td>
+   <td><code>1</code></td>
+   <td><code>[0, 1, 2, 3, 4]</code></td>
+   <td><code>11</code></td>
+  </tr>
+  <tr>
+   <th scope="row">third call</th>
+   <td><code>11</code></td>
+   <td><code>2</code></td>
+   <td><code>2</code></td>
+   <td><code>[0, 1, 2, 3, 4]</code></td>
+   <td><code>13</code></td>
+  </tr>
+  <tr>
+   <th scope="row">fourth call</th>
+   <td><code>13</code></td>
+   <td><code>3</code></td>
+   <td><code>3</code></td>
+   <td><code>[0, 1, 2, 3, 4]</code></td>
+   <td><code>16</code></td>
+  </tr>
+  <tr>
+   <th scope="row">fifth call</th>
+   <td><code>16</code></td>
+   <td><code>4</code></td>
+   <td><code>4</code></td>
+   <td><code>[0, 1, 2, 3, 4]</code></td>
+   <td><code>20</code></td>
+  </tr>
+ </tbody>
+</table>
+
+<p>这种情况下<code>reduce()</code>返回的值是<code>20</code>。</p>
+
+<h2 id="例子">例子</h2>
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+<h3 id="数组里所有值的和">数组里所有值的和</h3>
+
+<pre class="brush: js notranslate">var sum = [0, 1, 2, 3].reduce(function (accumulator, currentValue) {
+  return accumulator + currentValue;
+}, 0);
+// 和为 6</pre>
+
+<p>你也可以写成箭头函数的形式：</p>
+
+<pre class="brush: js notranslate">var total = [ 0, 1, 2, 3 ].reduce(
+  ( acc, cur ) =&gt; acc + cur,
+  0
+);</pre>
+
+<h3 id="累加对象数组里的值">累加对象数组里的值</h3>
+
+<p>要累加对象数组中包含的值，必须提供初始值，以便各个item正确通过你的函数。</p>
+
+<pre class="brush: js notranslate">var initialValue = 0;
+var sum = [{x: 1}, {x:2}, {x:3}].reduce(function (accumulator, currentValue) {
+    return accumulator + currentValue.x;
+},initialValue)
+
+console.log(sum) // logs 6</pre>
+
+<p>你也可以写成箭头函数的形式：</p>
+
+<pre class="brush: js notranslate">var initialValue = 0;
+var sum = [{x: 1}, {x:2}, {x:3}].reduce(
+    (accumulator, currentValue) =&gt; accumulator + currentValue.x
+    ,initialValue
+);
+
+console.log(sum) // logs 6
+</pre>
+
+<h3 id="将二维数组转化为一维">将二维数组转化为一维</h3>
+
+<pre class="brush: js notranslate">var flattened = [[0, 1], [2, 3], [4, 5]].reduce(
+  function(a, b) {
+    return a.concat(b);
+  },
+  []
+);
+// flattened is [0, 1, 2, 3, 4, 5]
+</pre>
+
+<p>你也可以写成箭头函数的形式：</p>
+
+<pre class="brush: js notranslate">var flattened = [[0, 1], [2, 3], [4, 5]].reduce(
+ ( acc, cur ) =&gt; acc.concat(cur),
+ []
+);
+
+</pre>
+
+<h3 id="计算数组中每个元素出现的次数">计算数组中每个元素出现的次数</h3>
+
+<pre class="brush: js notranslate">var names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];
+
+var countedNames = names.reduce(function (allNames, name) {
+  if (name in allNames) {
+    allNames[name]++;
+  }
+  else {
+    allNames[name] = 1;
+  }
+  return allNames;
+}, {});
+// countedNames is:
+// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }</pre>
+
+<h3 id="按属性对object分类">按属性对object分类</h3>
+
+<pre class="brush: js notranslate">var people = [
+  { name: 'Alice', age: 21 },
+  { name: 'Max', age: 20 },
+  { name: 'Jane', age: 20 }
+];
+
+function groupBy(objectArray, property) {
+  return objectArray.reduce(function (acc, obj) {
+    var key = obj[property];
+    if (!acc[key]) {
+      acc[key] = [];
+    }
+    acc[key].push(obj);
+    return acc;
+  }, {});
+}
+
+var groupedPeople = groupBy(people, 'age');
+// groupedPeople is:
+// {
+//   20: [
+//     { name: 'Max', age: 20 },
+//     { name: 'Jane', age: 20 }
+//   ],
+//   21: [{ name: 'Alice', age: 21 }]
+// }
+</pre>
+
+<h3 id="使用扩展运算符和initialValue绑定包含在对象数组中的数组">使用扩展运算符和initialValue绑定包含在对象数组中的数组</h3>
+
+<pre class="brush: js notranslate">// friends - 对象数组
+// where object field "books" - list of favorite books
+var friends = [{
+  name: 'Anna',
+  books: ['Bible', 'Harry Potter'],
+  age: 21
+}, {
+  name: 'Bob',
+  books: ['War and peace', 'Romeo and Juliet'],
+  age: 26
+}, {
+  name: 'Alice',
+  books: ['The Lord of the Rings', 'The Shining'],
+  age: 18
+}];
+
+// allbooks - list which will contain all friends' books +
+// additional list contained in initialValue
+var allbooks = friends.reduce(function(prev, curr) {
+  return [...prev, ...curr.books];
+}, ['Alphabet']);
+
+// allbooks = [
+//   'Alphabet', 'Bible', 'Harry Potter', 'War and peace',
+//   'Romeo and Juliet', 'The Lord of the Rings',
+//   'The Shining'
+// ]
+</pre>
+
+<h3 id="数组去重">数组去重</h3>
+
+<div class="blockIndicator note">
+<p>注意： 如果你正在使用一个可以兼容{{jsxref("Set")}} 和 {{jsxref("Array.from()")}} 的环境， 你可以使用<code>let orderedArray = Array.from(new Set(myArray));</code> 来获得一个相同元素被移除的数组。</p>
+</div>
+
+<pre class="brush: js notranslate">let myArray = ['a', 'b', 'a', 'b', 'c', 'e', 'e', 'c', 'd', 'd', 'd', 'd']
+let myOrderedArray = myArray.reduce(function (accumulator, currentValue) {
+  if (accumulator.indexOf(currentValue) === -1) {
+    accumulator.push(currentValue)
+  }
+  return accumulator
+}, [])
+
+console.log(myOrderedArray)</pre>
+
+<pre class="brush: js notranslate">let arr = [1,2,1,2,3,5,4,5,3,4,4,4,4];
+let result = arr.sort().reduce((init, current) =&gt; {
+    if(init.length === 0 || init[init.length-1] !== current) {
+        init.push(current);
+    }
+    return init;
+}, []);
+console.log(result); //[1,2,3,4,5]</pre>
+
+<h3 id="按顺序运行Promise">按顺序运行Promise</h3>
+
+<pre class="brush: js notranslate">/**
+ * Runs promises from array of functions that can return promises
+ * in chained manner
+ *
+ * @param {array} arr - promise arr
+ * @return {Object} promise object
+ */
+function runPromiseInSequence(arr, input) {
+  return arr.reduce(
+    (promiseChain, currentFunction) =&gt; promiseChain.then(currentFunction),
+    Promise.resolve(input)
+  );
+}
+
+// promise function 1
+function p1(a) {
+  return new Promise((resolve, reject) =&gt; {
+    resolve(a * 5);
+  });
+}
+
+// promise function 2
+function p2(a) {
+  return new Promise((resolve, reject) =&gt; {
+    resolve(a * 2);
+  });
+}
+
+// function 3  - will be wrapped in a resolved promise by .then()
+function f3(a) {
+ return a * 3;
+}
+
+// promise function 4
+function p4(a) {
+  return new Promise((resolve, reject) =&gt; {
+    resolve(a * 4);
+  });
+}
+
+const promiseArr = [p1, p2, f3, p4];
+runPromiseInSequence(promiseArr, 10)
+  .then(console.log);   // 1200
+</pre>
+
+<h3 id="功能型函数管道">功能型函数管道</h3>
+
+<pre class="brush: js notranslate">// Building-blocks to use for composition
+const double = x =&gt; x + x;
+const triple = x =&gt; 3 * x;
+const quadruple = x =&gt; 4 * x;
+
+// Function composition enabling pipe functionality
+const pipe = (...functions) =&gt; input =&gt; functions.reduce(
+    (acc, fn) =&gt; fn(acc),
+    input
+);
+
+// Composed functions for multiplication of specific values
+const multiply6 = pipe(double, triple);
+const multiply9 = pipe(triple, triple);
+const multiply16 = pipe(quadruple, quadruple);
+const multiply24 = pipe(double, triple, quadruple);
+
+// Usage
+multiply6(6); // 36
+multiply9(9); // 81
+multiply16(16); // 256
+multiply24(10); // 240
+</pre>
+
+<h3 id="使用_reduce实现map">使用 reduce实现map</h3>
+
+<pre class="brush: js notranslate">if (!Array.prototype.mapUsingReduce) {
+  Array.prototype.mapUsingReduce = function(callback, thisArg) {
+    return this.reduce(function(mappedArray, currentValue, index, array) {
+      mappedArray[index] = callback.call(thisArg, currentValue, index, array)
+      return mappedArray
+    }, [])
+  }
+}
+
+[1, 2, , 3].mapUsingReduce(
+  (currentValue, index, array) =&gt; currentValue + index + array.length
+) // [5, 7, , 10]
+</pre>
+
+<h2 id="Polyfill">Polyfill</h2>
+
+<pre class="brush: js notranslate">// Production steps of ECMA-262, Edition 5, 15.4.4.21
+// Reference: http://es5.github.io/#x15.4.4.21
+// https://tc39.github.io/ecma262/#sec-array.prototype.reduce
+if (!Array.prototype.reduce) {
+  Object.defineProperty(Array.prototype, 'reduce', {
+    value: function(callback /*, initialValue*/) {
+      if (this === null) {
+        throw new TypeError( 'Array.prototype.reduce ' +
+          'called on null or undefined' );
+      }
+      if (typeof callback !== 'function') {
+        throw new TypeError( callback +
+          ' is not a function');
+      }
+
+      // 1. Let O be ? ToObject(this value).
+      var o = Object(this);
+
+      // 2. Let len be ? ToLength(? Get(O, "length")).
+      var len = o.length &gt;&gt;&gt; 0;
+
+      // Steps 3, 4, 5, 6, 7
+      var k = 0;
+      var value;
+
+      if (arguments.length &gt;= 2) {
+        value = arguments[1];
+      } else {
+        while (k &lt; len &amp;&amp; !(k in o)) {
+          k++;
+        }
+
+        // 3. If len is 0 and initialValue is not present,
+        //    throw a TypeError exception.
+        if (k &gt;= len) {
+          throw new TypeError( 'Reduce of empty array ' +
+            'with no initial value' );
+        }
+        value = o[k++];
+      }
+
+      // 8. Repeat, while k &lt; len
+      while (k &lt; len) {
+        // a. Let Pk be ! ToString(k).
+        // b. Let kPresent be ? HasProperty(O, Pk).
+        // c. If kPresent is true, then
+        //    i.  Let kValue be ? Get(O, Pk).
+        //    ii. Let accumulator be ? Call(
+        //          callbackfn, undefined,
+        //          « accumulator, kValue, k, O »).
+        if (k in o) {
+          value = callback(value, o[k], k, o);
+        }
+
+        // d. Increase k by 1.
+        k++;
+      }
+
+      // 9. Return accumulator.
+      return value;
+    }
+  });
+}
+</pre>
+
+<p>如果您需要兼容不支持<code><a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty">Object.defineProperty</a></code>的JavaScript引擎，那么最好不要 polyfill <code>Array.prototype</code>方法，因为你无法使其成为<strong>不可枚举</strong>的。</p>
+
+<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>{{SpecName('ES5.1', '#sec-15.4.4.21', 'Array.prototype.reduce')}}</td>
+   <td>{{Spec2('ES5.1')}}</td>
+   <td>初始定语. 实施于 JavaScript 1.8.</td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES6', '#sec-array.prototype.reduce', 'Array.prototype.reduce')}}</td>
+   <td>{{Spec2('ES6')}}</td>
+   <td></td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ESDraft', '#sec-array.prototype.reduce', 'Array.prototype.reduce')}}</td>
+   <td>{{Spec2('ESDraft')}}</td>
+   <td></td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+<div>
+
+
+<p>{{Compat("javascript.builtins.Array.reduce")}}</p>
+</div>
+
+<h2 id="相关链接">相关链接</h2>
+
+<ul>
+ <li>{{jsxref("Array.prototype.reduceRight()")}}</li>
+</ul>