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authorPeter Bengtsson <mail@peterbe.com>2020-12-08 14:40:17 -0500
committerPeter Bengtsson <mail@peterbe.com>2020-12-08 14:40:17 -0500
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+---
+title: Array.prototype.find()
+slug: Web/JavaScript/Reference/Global_Objects/Array/find
+tags:
+ - ECMAScript6
+ - JavaScript
+ - polyfill
+ - 数组
+ - 方法
+translation_of: Web/JavaScript/Reference/Global_Objects/Array/find
+---
+<div>{{JSRef}}</div>
+
+<p> <code><strong>find()</strong></code> 方法返回数组中满足提供的测试函数的第一个元素的值。否则返回 {{jsxref("undefined")}}。</p>
+
+<p>{{EmbedInteractiveExample("pages/js/array-find.html")}}</p>
+
+<p>另请参见  {{jsxref("Array.findIndex", "findIndex()")}} 方法,它返回数组中找到的元素的索引,而不是其值。</p>
+
+<p>如果你需要找到一个元素的位置或者一个元素是否存在于数组中,使用{{jsxref("Array.prototype.indexOf()")}} 或 {{jsxref("Array.prototype.includes()")}}。</p>
+
+<h2 id="Syntax" name="Syntax">语法</h2>
+
+<pre class="syntaxbox"><code><em>arr</em>.find(<em>callback</em>[, <em>thisArg</em>])</code></pre>
+
+<h3 id="Parameters" name="Parameters">参数</h3>
+
+<dl>
+ <dt><code>callback</code></dt>
+ <dd>在数组每一项上执行的函数,接收 3 个参数:
+ <dl>
+ <dt><code>element</code></dt>
+ <dd>当前遍历到的元素。</dd>
+ <dt><code>index</code>{{optional_inline}}</dt>
+ <dd>当前遍历到的索引。</dd>
+ <dt><code>array</code>{{optional_inline}}</dt>
+ <dd>数组本身。</dd>
+ </dl>
+ </dd>
+ <dt><code>thisArg</code>{{Optional_inline}}</dt>
+ <dd>执行回调时用作<code>this</code> 的对象。</dd>
+</dl>
+
+<h3 id="返回值">返回值</h3>
+
+<p>数组中第一个满足所提供测试函数的元素的值,否则返回 {{jsxref("undefined")}}。</p>
+
+<h2 id="Description" name="Description">描述</h2>
+
+<p><code>find</code>方法对数组中的每一项元素执行一次 <code>callback</code> 函数,直至有一个 callback 返回 <code>true</code>。当找到了这样一个元素后,该方法会立即返回这个元素的值,否则返回 {{jsxref("undefined")}}。注意 <code>callback </code>函数会为数组中的每个索引调用即从 <code>0 </code>到 <code>length - 1</code>,而不仅仅是那些被赋值的索引,这意味着对于稀疏数组来说,该方法的效率要低于那些只遍历有值的索引的方法。</p>
+
+<p><code>callback</code>函数带有3个参数:当前元素的值、当前元素的索引,以及数组本身。</p>
+
+<p>如果提供了 <code>thisArg</code>参数,那么它将作为每次 <code>callback</code>函数执行时的<code>this</code> ,如果未提供,则使用 {{jsxref("undefined")}}。</p>
+
+<p><code>find</code>方法不会改变数组。</p>
+
+<p>在第一次调用 <code>callback</code>函数时会确定元素的索引范围,因此在 <code>find</code>方法开始执行之后添加到数组的新元素将不会被 <code>callback</code>函数访问到。如果数组中一个尚未被<code>callback</code>函数访问到的元素的值被<code>callback</code>函数所改变,那么当<code>callback</code>函数访问到它时,它的值是将是根据它在数组中的索引所访问到的当前值。被删除的元素仍旧会被访问到,但是其值已经是undefined了。</p>
+
+<h2 id="示例">示例</h2>
+
+<h3 id="用对象的属性查找数组里的对象">用对象的属性查找数组里的对象</h3>
+
+<pre class="brush: js">var inventory = [
+ {name: 'apples', quantity: 2},
+ {name: 'bananas', quantity: 0},
+ {name: 'cherries', quantity: 5}
+];
+
+function findCherries(fruit) {
+ return fruit.name === 'cherries';
+}
+
+console.log(inventory.find(findCherries)); // { name: 'cherries', quantity: 5 }</pre>
+
+<h3 id="寻找数组中的质数">寻找数组中的质数</h3>
+
+<p>下面的例子展示了如何从一个数组中寻找质数(如果找不到质数则返回{{jsxref("undefined")}})</p>
+
+<pre class="brush: js">function isPrime(element, index, array) {
+ var start = 2;
+ while (start &lt;= Math.sqrt(element)) {
+ if (element % start++ &lt; 1) {
+ return false;
+ }
+ }
+ return element &gt; 1;
+}
+
+console.log([4, 6, 8, 12].find(isPrime)); // undefined, not found
+console.log([4, 5, 8, 12].find(isPrime)); // 5
+
+</pre>
+
+<p>当在回调中删除数组中的一个值时,当访问到这个位置时,其传入的值是 undefined:</p>
+
+<pre class="brush: js">// Declare array with no element at index 2, 3 and 4
+var a = [0,1,,,,5,6];
+
+// Shows all indexes, not just those that have been assigned values
+a.find(function(value, index) {
+ console.log('Visited index ' + index + ' with value ' + value);
+});
+
+// Shows all indexes, including deleted
+a.find(function(value, index) {
+
+ // Delete element 5 on first iteration
+ if (index == 0) {
+ console.log('Deleting a[5] with value ' + a[5]);
+ delete a[5]; // 注:这里只是将a[5]设置为undefined,可以试试用a.pop()删除最后一项,依然会遍历到被删的那一项
+ }
+ // Element 5 is still visited even though deleted
+ console.log('Visited index ' + index + ' with value ' + value);
+});</pre>
+
+<h2 id="Polyfill" name="Polyfill">Polyfill</h2>
+
+<p>本方法在ECMAScript 6规范中被加入,可能不存在于某些实现中。你可以通过以下代码来补充 <code>Array.prototype.find()</code>。</p>
+
+<pre class="brush: js">// https://tc39.github.io/ecma262/#sec-array.prototype.find
+if (!Array.prototype.find) {
+ Object.defineProperty(Array.prototype, 'find', {
+ value: function(predicate) {
+ // 1. Let O be ? ToObject(this value).
+ if (this == null) {
+ throw new TypeError('"this" is null or not defined');
+ }
+
+ var o = Object(this);
+
+ // 2. Let len be ? ToLength(? Get(O, "length")).
+ var len = o.length &gt;&gt;&gt; 0;
+
+ // 3. If IsCallable(predicate) is false, throw a TypeError exception.
+ if (typeof predicate !== 'function') {
+ throw new TypeError('predicate must be a function');
+ }
+
+ // 4. If thisArg was supplied, let T be thisArg; else let T be undefined.
+ var thisArg = arguments[1];
+
+ // 5. Let k be 0.
+ var k = 0;
+
+ // 6. Repeat, while k &lt; len
+ while (k &lt; len) {
+ // a. Let Pk be ! ToString(k).
+ // b. Let kValue be ? Get(O, Pk).
+ // c. Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
+ // d. If testResult is true, return kValue.
+ var kValue = o[k];
+ if (predicate.call(thisArg, kValue, k, o)) {
+ return kValue;
+ }
+ // e. Increase k by 1.
+ k++;
+ }
+
+ // 7. Return undefined.
+ return undefined;
+ }
+ });
+}</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>{{SpecName('ES6', '#sec-array.prototype.find', 'Array.prototype.find')}}</td>
+ <td>{{Spec2('ES6')}}</td>
+ <td>Initial definition.</td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ESDraft', '#sec-array.prototype.find', 'Array.prototype.find')}}</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>
+
+<p>{{Compat("javascript.builtins.Array.find")}}</p>
+
+<h2 id="See_also" name="See_also">相关链接</h2>
+
+<ul>
+ <li>{{jsxref("Array.prototype.findIndex()")}} – find and return an index</li>
+ <li>{{jsxref("Array.prototype.includes()")}} – test whether a value exists in the array</li>
+ <li>{{jsxref("Array.prototype.filter()")}} – find all matching elements</li>
+ <li>{{jsxref("Array.prototype.every()")}} – test all elements together</li>
+ <li>{{jsxref("Array.prototype.some()")}} – test at least one element</li>
+</ul>