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

<article id="wikiArticle">
<div></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 <a href="Reference/Global_Objects/undefined" title="undefined是全局对象的一个属性。也就是说，它是全局作用域的一个变量。undefined的最初值就是原始数据类型undefined。"><code>undefined</code></a>。</p>
<p><iframe class="interactive interactive-js" frameborder="0" height="250" src="https://interactive-examples.mdn.mozilla.net/pages/js/array-find.html" width="100%"></iframe></p>
<p>另请参见  <a href="Reference/Global_Objects/Array/findIndex" title="findIndex()方法返回数组中满足提供的测试函数的第一个元素的索引。否则返回-1。"><code>findIndex()</code></a> 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<a href="Reference/Global_Objects/Array/indexOf" title="indexOf()方法返回在数组中可以找到一个给定元素的第一个索引，如果不存在，则返回-1。"><code>Array.prototype.indexOf()</code></a> 或 <a href="Reference/Global_Objects/Array/includes" title="includes() 方法用来判断一个数组是否包含一个指定的值，根据情况，如果包含则返回 true，否则返回false。"><code>Array.prototype.includes()</code></a>。</p>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre><code class="language-javascript"><code><em>arr</em>.find(<em>callback</em>[, <em>thisArg</em>])</code></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><span class="inlineIndicator optional optionalInline">可选</span></dt>
<dd>当前遍历到的索引。</dd>
<dt><code>array</code><span class="inlineIndicator optional optionalInline">可选</span></dt>
<dd>数组本身。</dd>
</dl>
</dd>
<dt><code>thisArg</code><span class="inlineIndicator optional optionalInline">可选</span></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 <a href="Reference/Global_Objects/undefined" title="undefined是全局对象的一个属性。也就是说，它是全局作用域的一个变量。undefined的最初值就是原始数据类型undefined。"><code>undefined</code></a>。</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 <a href="Reference/Global_Objects/undefined" title="undefined是全局对象的一个属性。也就是说，它是全局作用域的一个变量。undefined的最初值就是原始数据类型undefined。"><code>undefined</code></a>。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 <a href="Reference/Global_Objects/undefined" title="undefined是全局对象的一个属性。也就是说，它是全局作用域的一个变量。undefined的最初值就是原始数据类型undefined。"><code>undefined</code></a>。</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。</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><code  class="language-javascript">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 }</code></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<a href="Reference/Global_Objects/undefined" title="undefined是全局对象的一个属性。也就是说，它是全局作用域的一个变量。undefined的最初值就是原始数据类型undefined。"><code>undefined</code></a>）</p>
<pre><code  class="language-javascript">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

</code></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，其传入的值时 undefined：</p>
<pre><code  class="language-javascript">// 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); 
});</code></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><code  class="language-javascript">// 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;
    }
  });
}</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-262/6.0/#sec-array.prototype.find" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Array.prototype.find</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition.</td>
</tr>
<tr>
<td><a class="external" href="https://tc39.github.io/ecma262/#sec-array.prototype.find" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Array.prototype.find</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>
<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"><code>find</code></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>
              45</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>
              25</td><td class="bc-supports-no bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-no only-icon" title="No support">
<span>No support</span>
</abbr>
              No</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>
              32</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>
              8</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>
              8</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 bc-has-history"><span class="bc-browser-name">nodejs</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              4.0.0<div class="bc-icons"></div><section class="bc-history" id="sect1"><dl><dt class="bc-supports-yes bc-supports"><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              4.0.0<div class="bc-icons"></div></dt><dd></dd><dt class="bc-supports-yes bc-supports"><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              0.12<div class="bc-icons"><abbr class="only-icon" title="User must explicitly enable this feature."><span>Disabled</span><i class="ic-disabled"></i></abbr> </div></dt><dd><abbr class="only-icon" title="User must explicitly enable this feature."><span>Disabled</span><i class="ic-disabled"></i></abbr> From version 0.12: this feature is behind the <code>--harmony</code> runtime flag.</dd></dl></section></td></tr></tbody></table><section class="bc-legend" id="sect2"><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><dt><span class="bc-supports-no bc-supports">
<abbr class="bc-level bc-level-no only-icon" title="No support">
<span>No support</span>
                  
                </abbr></span></dt><dd>No support</dd><dt><abbr class="only-icon" title="User must explicitly enable this feature."><span>User must explicitly enable this feature.</span><i class="ic-disabled"></i></abbr></dt><dd>User must explicitly enable this feature.</dd></dl></section></div><p></p>
<h2 id="See_also" name="See_also">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/Array/findIndex" title="findIndex()方法返回数组中满足提供的测试函数的第一个元素的索引。否则返回-1。"><code>Array.prototype.findIndex()</code></a> – find and return an index</li>
<li><a href="Reference/Global_Objects/Array/includes" title="includes() 方法用来判断一个数组是否包含一个指定的值，根据情况，如果包含则返回 true，否则返回false。"><code>Array.prototype.includes()</code></a> – test whether a value exists in the array</li>
<li><a href="Reference/Global_Objects/Array/filter" title="filter() 方法创建一个新数组, 其包含通过所提供函数实现的测试的所有元素。"><code>Array.prototype.filter()</code></a> – find all matching elements</li>
<li><a href="Reference/Global_Objects/Array/every" title="every() 方法测试数组的所有元素是否都通过了指定函数的测试。"><code>Array.prototype.every()</code></a> – test all elements together</li>
<li><a href="Reference/Global_Objects/Array/some" title="some() 方法测试是否至少有一个元素通过由提供的函数实现的测试。"><code>Array.prototype.some()</code></a> – test at least one element</li>
</ul>
</article>