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author | Peter Bengtsson <mail@peterbe.com> | 2020-12-08 14:40:17 -0500 |
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committer | Peter Bengtsson <mail@peterbe.com> | 2020-12-08 14:40:17 -0500 |
commit | 33058f2b292b3a581333bdfb21b8f671898c5060 (patch) | |
tree | 51c3e392513ec574331b2d3f85c394445ea803c6 /files/zh-cn/web/javascript/reference/global_objects/array/find | |
parent | 8b66d724f7caf0157093fb09cfec8fbd0c6ad50a (diff) | |
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diff --git a/files/zh-cn/web/javascript/reference/global_objects/array/find/index.html b/files/zh-cn/web/javascript/reference/global_objects/array/find/index.html new file mode 100644 index 0000000000..425901f14d --- /dev/null +++ b/files/zh-cn/web/javascript/reference/global_objects/array/find/index.html @@ -0,0 +1,200 @@ +--- +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 <= Math.sqrt(element)) { + if (element % start++ < 1) { + return false; + } + } + return element > 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 >>> 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 < len + while (k < 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> |