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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
+---
+<div>{{JSRef}}</div>
+
+<p><code><strong>reduce()</strong></code> 方法將一個累加器及陣列中每項元素（由左至右）傳入回呼函式，將陣列化為單一值。</p>
+
+<div>{{EmbedInteractiveExample("pages/js/array-reduce.html")}}</div>
+
+<h2 id="語法">語法</h2>
+
+<pre class="syntaxbox"><var>arr</var>.reduce(<var>callback[accumulator, currentValue, currentIndex, array], </var><var>initialValue</var>)</pre>
+
+<h3 id="參數">參數</h3>
+
+<dl>
+ <dt><code>callback</code></dt>
+ <dd>用於處理陣列中每個元素的函式，可傳入四個參數：
+ <dl>
+  <dt><code>accumulator</code></dt>
+  <dd>用來累積回呼函式回傳值的累加器（accumulator）或 <code>initialValue</code>（若有提供的話，詳如下敘）。累加器是上一次呼叫後，所回傳的累加數值。</dd>
+  <dt><code>currentValue</code></dt>
+  <dd>原陣列目前所迭代處理中的元素。</dd>
+  <dt><code>currentIndex</code>{{optional_inline}}</dt>
+  <dd>原陣列目前所迭代處理中的元素之索引。若有傳入 <code>initialValue</code>，則由索引 0 之元素開始，若無則自索引 1 之元素開始。</dd>
+  <dt><code>array</code>{{optional_inline}}</dt>
+  <dd>呼叫 <code>reduce()</code> 方法的陣列。</dd>
+ </dl>
+ </dd>
+ <dt><code>initialValue</code>{{optional_inline}}</dt>
+ <dd>於第一次呼叫 <code>callback</code> 時要傳入的累加器初始值。若沒有提供初始值，則原陣列的第一個元素將會被當作初始的累加器。假如於一個空陣列呼叫 <code>reduce()</code> 方法且沒有提供累加器初始值，將會發生錯誤。</dd>
+</dl>
+
+<h3 id="回傳值">回傳值</h3>
+
+<p>簡化後的結果值。</p>
+
+<h2 id="描述">描述</h2>
+
+<p><code>reduce()</code> 會對每一個目前迭代到的陣列元素（除了空值以外）執行 <code>callback</code> 函式，回呼函式會接收四個參數：</p>
+
+<ul>
+ <li><code>accumulator</code></li>
+ <li><code>currentValue</code></li>
+ <li><code>currentIndex</code></li>
+ <li><code>array</code></li>
+</ul>
+
+<p>當回呼函式第一次被呼叫時，<code>accumulator</code> 與 <code>currentValue</code> 的值可能為兩種不同的狀況：若在呼叫 <code>reduce()</code> 時有提供 <code>initialValue</code>，則 <code>accumulator</code> 將會等於 <code>initialValue</code>，且 <code>currentValue</code> 會等於陣列中的第一個元素值；若沒有提供 <code>initialValue</code>，則 <code>accumulator</code> 會等於陣列的第一個元素值，且 <code>currentValue</code> 將會等於陣列的第二個元素值。</p>
+
+<div class="note">
+<p><strong>備註：</strong>假如 <code>initialValue</code> 未被提供，<code>reduce()</code> 將會跳過第一個陣列索引，從陣列索引 1 開始執行回呼函式。若有提供 <code>initialValue</code>，則會由陣列索引 0 開始執行。</p>
+</div>
+
+<p>若陣列為空且沒有提供 <code>initialValue</code>，將會拋出 {{jsxref("TypeError")}}。假如陣列只有一個元素（無論其索引位置為何）並且沒有提供 <code>initialValue</code>，或如果提供了 <code>initialValue</code> 但陣列為空，則此唯一的值將會被直接回傳<em>而不會呼叫 <code>callback</code> 函式</em>。</p>
+
+<p>提供累加器初始值通常較為安全，因為在沒有傳入 <code>initialValue</code> 的情況下會有三種可能的輸出結果，如下列範例：</p>
+
+<pre class="brush: js">var maxCallback = ( acc, cur ) =&gt; Math.max( acc.x, cur.x );
+var maxCallback2 = ( max, cur ) =&gt; Math.max( max, cur );
+
+// reduce() without initialValue
+[ { x: 22 }, { x: 42 } ].reduce( maxCallback ); // 42
+[ { x: 22 }            ].reduce( maxCallback ); // { x: 22 }
+[                      ].reduce( maxCallback ); // TypeError
+
+// map/reduce; better solution, also works for empty or larger arrays
+[ { x: 22 }, { x: 42 } ].map( el =&gt; el.x )
+                        .reduce( maxCallback2, -Infinity );
+</pre>
+
+<h3 id="reduce()_如何運作">reduce() 如何運作</h3>
+
+<p>假設 <code>reduce()</code> 以下例方式使用：</p>
+
+<pre class="brush: js">[0, 1, 2, 3, 4].reduce(
+  function (
+<code>    accumulator,</code>
+    <code>currentValue</code>,
+    <code>currentIndex</code>,
+    array
+  ) {
+    return <code>accumulator</code> + currentValue;
+  }
+);
+</pre>
+
+<p>所傳入的回呼函式將被呼叫四次，所傳入的參數與回傳值如下所示：</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>
+
+<p><code>reduce()</code> 的最終回傳值將會是最後一次呼叫回呼函式的回傳值 (<code>10</code>)。</p>
+
+<p>你也可以傳入一個{{jsxref("Functions/Arrow_functions", "箭頭函式","",1)}}來替代一個完整的函式。下方的程式碼執行的結果將與前述例子相同。</p>
+
+<pre class="brush: js">[0, 1, 2, 3, 4].reduce( (prev, curr) =&gt; prev + curr );
+</pre>
+
+<p>如果你有提供第二個參數值給 <code>reduce()</code>，執行的結果如下：</p>
+
+<pre class="brush: js">[0, 1, 2, 3, 4].reduce(
+  (<code>accumulator</code>, currentValue, currentIndex, array) =&gt; {
+    return <code>accumulator</code> + 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><code>reduce()</code> 執行的結果將會是 <code>20</code>。</p>
+
+<h2 id="範例">範例</h2>
+
+<h3 id="加總所有陣例之元素值">加總所有陣例之元素值</h3>
+
+<pre class="brush: js">var sum = [0, 1, 2, 3].reduce(function (a, b) {
+  return a + b;
+}, 0);
+// sum is 6
+</pre>
+
+<p>另外，也可以寫成箭頭函式：</p>
+
+<pre class="brush: js">var total = [ 0, 1, 2, 3 ].reduce(
+  ( acc, cur ) =&gt; acc + cur,
+  0
+);</pre>
+
+<h3 id="攤平一個多維陣列">攤平一個多維陣列</h3>
+
+<pre class="brush: js">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>另外，也可以寫成箭頭函式：</p>
+
+<pre class="brush: js">var flattened = [[0, 1], [2, 3], [4, 5]].reduce(
+  ( acc, cur ) =&gt; acc.concat(cur),
+  []
+);
+</pre>
+
+<h3 id="計算相同元素數量並以物件鍵值顯示">計算相同元素數量並以物件鍵值顯示</h3>
+
+<pre class="brush: js">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="使用_spread_運算子與給定初始值，結合物件中的陣列元素">使用 spread 運算子與給定初始值，結合物件中的陣列元素</h3>
+
+<pre class="brush: js">// friends - an array of objects
+// 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>
+
+<p> </p>
+
+<h3 id="移除陣列中的重複項目">移除陣列中的重複項目</h3>
+
+<pre class="brush: js">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="序列執行_Promise">序列執行 Promise</h3>
+
+<pre class="brush: js">/**
+ * Runs promises from promise array in chained manner
+ *
+ * @param {array} arr - promise arr
+ * @return {Object} promise object
+ */
+function runPromiseInSequense(arr) {
+  return arr.reduce((promiseChain, currentPromise) =&gt; {
+    return promiseChain.then((chainedResult) =&gt; {
+      return currentPromise(chainedResult)
+        .then((res) =&gt; res)
+    })
+  }, Promise.resolve());
+}
+
+// promise function 1
+function p1() {
+  return new Promise((resolve, reject) =&gt; {
+    resolve(5);
+  });
+}
+
+// promise function 2
+function p2(a) {
+  return new Promise((resolve, reject) =&gt; {
+    resolve(a * 2);
+  });
+}
+
+// promise function 3
+function p3(a) {
+  return new Promise((resolve, reject) =&gt; {
+    resolve(a * 3);
+  });
+}
+
+const promiseArr = [p1, p2, p3];
+runPromiseInSequense(promiseArr)
+  .then((res) =&gt; {
+    console.log(res);   // 30
+  });
+
+</pre>
+
+<h2 id="Polyfill">Polyfill</h2>
+
+<pre class="brush: js">// 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>如果還需要支援老舊到不支援 <code><a href="/zh-TW/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty">Object.defineProperty</a></code> 的 JavaScript 引擎，最好不要 polyfill <code>Array.prototype</code> 方法，因為你無法令其不可枚舉（non-enumerable）。</p>
+
+<h2 id="規範">規範</h2>
+
+<table class="standard-table">
+ <tbody>
+  <tr>
+   <th scope="col">規範</th>
+   <th scope="col">狀態</th>
+   <th scope="col">註解</th>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES5.1', '#sec-15.4.4.21', 'Array.prototype.reduce()')}}</td>
+   <td>{{Spec2('ES5.1')}}</td>
+   <td>Initial definition. Implemented in 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="瀏覽器相容性">瀏覽器相容性</h2>
+
+<div>
+
+
+<p>{{Compat("javascript.builtins.Array.reduce")}}</p>
+</div>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li>{{jsxref("Array.prototype.reduceRight()")}}</li>
+</ul>