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---
title: 'Document: DOMContentLoaded 事件'
slug: Web/API/Document/DOMContentLoaded_event
tags:
- 事件
translation_of: Web/API/Document/DOMContentLoaded_event
---
<div>{{APIRef}}</div>
<p>当纯HTML被完全加载以及解析时，<strong><code>DOMContentLoaded</code> </strong>事件会被触发，而不必等待样式表，图片或者子框架完成加载。</p>
<table class="properties">
<tbody>
<tr>
<th scope="row">冒泡阶段</th>
<td>Yes</td>
</tr>
<tr>
<th scope="row">可撤销性</th>
<td>Yes (尽管它被指定为一个简单事件时是不可撤销的)</td>
</tr>
<tr>
<th scope="row">接口</th>
<td>{{domxref("Event")}}</td>
</tr>
<tr>
<th scope="row">事件句柄属性</th>
<td>None</td>
</tr>
</tbody>
</table>
<p>一个易混用但不同的事件是，<code><a href="/en-US/docs/Web/API/Window/load_event">load</a></code>,这个事件仅仅应该在探测到整个页面完全加载完成时被使用。一个常见的错误就是在该使用<code>DOMContentLoaded</code>的地方使用了<code>load</code> 。</p>
<p>JavaScript的同步模式会导致DOM解析暂停。如果你想在用户请求页面时，首先尽可能先解析DOM，此时你可以使用<a href="/en-US/docs/Web/API/XMLHttpRequest/Synchronous_and_Asynchronous_Requests">JavaScript异步模式</a>，并且<a href="https://developers.google.com/speed/docs/insights/OptimizeCSSDelivery">优化样式表的加载</a>。在通常模式的加载过程中，样式表的加载会与DOM解析并行，从而迟缓主要HTMl文档的加载。</p>
<h2 id="例子"><strong>例子</strong></h2>
<h3 id="基本用法">基本用法</h3>
<pre class="brush: js">document.addEventListener('DOMContentLoaded', (event) => {
console.log('DOM fully loaded and parsed'); // 译者注："DOM完全加载以及解析"
});
</pre>
<h3 id="延迟_DOMContentLoaded"> 延迟 DOMContentLoaded</h3>
<pre class="brush: html"><script>
document.addEventListener('DOMContentLoaded', (event) => {
console.log('DOM fully loaded and parsed');
});
for( let i = 0; i < 1000000000; i++)
{} // 这段同步脚本将会延迟DOM解析,
// 所以DOMContentLoaded事件将会延迟执行.
</script>
</pre>
<h3 id="检查加载是否已经完成">检查加载是否已经完成</h3>
<p>在你的脚本有机会运行前，<code>DOMContentLoaded</code>可能就已经被触发。所以你在决定添加一个事件监听器前最好先检查一下。</p>
<pre class="brush: js">function doSomething() {
console.info('DOM loaded');
}
if (document.readyState === 'loading') { // 此时加载尚未完成
document.addEventListener('DOMContentLoaded', doSomething);
} else { // 此时`DOMContentLoaded` 已经被触发
doSomething();
}
</pre>
<h3 id="实例">实例</h3>
<h4 id="HTML">HTML</h4>
<pre class="brush: html"><div class="controls">
<button id="reload" type="button">Reload</button>
</div>
<div class="event-log">
<label>Event log:</label>
<textarea readonly class="event-log-contents" rows="8" cols="30"></textarea>
</div></pre>
<div class="hidden">
<h4 id="CSS">CSS</h4>
<pre class="brush: css">body {
display: grid;
grid-template-areas: "control log";
}
.controls {
grid-area: control;
display: flex;
align-items: center;
justify-content: center;
}
.event-log {
grid-area: log;
}
.event-log-contents {
resize: none;
}
label, button {
display: block;
}
#reload {
height: 2rem;
}
</pre>
</div>
<h4 id="JS">JS</h4>
<pre class="brush: js">const log = document.querySelector('.event-log-contents');
const reload = document.querySelector('#reload');
reload.addEventListener('click', () => {
log.textContent ='';
window.setTimeout(() => {
window.location.reload(true);
}, 200);
});
window.addEventListener('load', (event) => {
log.textContent = log.textContent + 'load\n';
});
document.addEventListener('readystatechange', (event) => {
log.textContent = log.textContent + `readystate: ${document.readyState}\n`;
});
document.addEventListener('DOMContentLoaded', (event) => {
log.textContent = log.textContent + `DOMContentLoaded\n`;
});
</pre>
<h4 id="结果展示">结果展示</h4>
<p>{{ EmbedLiveSample('实例', '100%', '160px') }}</p>
<h2 id="规范"><strong>规范</strong></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('HTML WHATWG', 'parsing.html#the-end:event-domcontentloaded', 'DOMContentLoaded')}}</td>
<td>{{Spec2('HTML WHATWG')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('HTML5 W3C', 'parsing.html#the-end:event-domcontentloaded', 'DOMContentLoaded')}}</td>
<td>{{Spec2('HTML5 W3C')}}</td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p>{{Compat("api.Document.DOMContentLoaded_event")}}</p>
<h2 id="相关链接">相关链接</h2>
<ul>
<li>有关事件: <code><a href="/en-US/docs/Web/API/Window/load_event">load</a></code>, <code><a href="/en-US/docs/Web/API/Document/readystatechange_event">readystatechange</a></code>, <code><a href="/en-US/docs/Web/API/Window/beforeunload_event">beforeunload</a></code>, <code><a href="/en-US/docs/Web/API/Window/unload_event">unload</a></code></li>
<li><code><a href="/en-US/docs/Web/API/Window">Window</a></code> 对象中的此事件: <code><a href="/en-US/docs/Web/API/Window/DOMContentLoaded_event">DOMContentLoaded</a></code></li>
</ul>
|
