--- 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 ) => Math.max( acc.x, cur.x ); var maxCallback2 = ( max, cur ) => 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 => 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) => 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) => { 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> <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 ) => 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) => 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 ) => 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) => { 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) => promiseChain.then(currentFunction), Promise.resolve(input) ); } // promise function 1 function p1(a) { return new Promise((resolve, reject) => { resolve(a * 5); }); } // promise function 2 function p2(a) { return new Promise((resolve, reject) => { 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) => { 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 => x + x; const triple = x => 3 * x; const quadruple = x => 4 * x; // Function composition enabling pipe functionality const pipe = (...functions) => input => functions.reduce( (acc, fn) => 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) => 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 >>> 0; // Steps 3, 4, 5, 6, 7 var k = 0; var value; if (arguments.length >= 2) { value = arguments[1]; } else { while (k < len && !(k in o)) { k++; } // 3. If len is 0 and initialValue is not present, // throw a TypeError exception. if (k >= len) { throw new TypeError( 'Reduce of empty array ' + 'with no initial value' ); } value = o[k++]; } // 8. Repeat, while k < len while (k < 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>