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+---
+title: 'Element:  键盘按下事件'
+slug: Web/API/Element/keydown_event
+translation_of: Web/API/Element/keydown_event
+---
+<div>{{APIRef}}</div>
+
+<div></div>
+
+<p><code><strong>keydown</strong></code>事件触发于键盘按键按下的时候。</p>
+
+<p>与{{Event("keypress")}} 事件不同的是, 所有按键均会触发<code>keydown</code>事件，无论这些按键是否会产生字符值。</p>
+
+<table class="properties">
+ <thead>
+ </thead>
+ <tbody>
+  <tr>
+   <th>Bubbles</th>
+   <td>Yes</td>
+  </tr>
+  <tr>
+   <th>Cancelable</th>
+   <td>Yes</td>
+  </tr>
+  <tr>
+   <th>Interface</th>
+   <td>{{domxref("KeyboardEvent")}}</td>
+  </tr>
+  <tr>
+   <th>Event handler property</th>
+   <td>{{domxref("GlobalEventHandlers.onkeydown", "onkeydown")}}</td>
+  </tr>
+ </tbody>
+</table>
+
+<p><code>keydown</code> 与 <code><a href="/en-US/docs/Web/API/Element/keyup_event">keyup</a></code> 事件捕获了键盘按键的操作，而 <code>keypress</code> 反映了具体输入某个字符的值。比如, 小写"a" 在<code>keydown</code> 和 <code>keyup</code>事件中输出的是大写A的Unicode编码65，但是在<code>keypress</code>中输出的就是小写"a"的Unicode编码97。大写 "A"在这些事件中输出的都是Unicode编码65。</p>
+
+<p>键盘事件只能由 <code>&lt;inputs&gt;</code>, <code>&lt;textarea&gt;</code> 以及任何具有  <code>contentEditable</code> 或 <code>tabindex="-1"</code>属性的组件触发。</p>
+
+<p>自 Firefox 65起,  <code>keydown</code> 与 <code><a href="/en-US/docs/Web/API/Element/keyup_event">keyup</a></code> 事件会在IME（输入法编辑器）复合事件中被触发,目的是为了提升CJKT（中日韩台地区）用户跨浏览器性能, ({{bug(354358)}}，更多详情访问 <a href="https://www.fxsitecompat.com/en-CA/docs/2018/keydown-and-keyup-events-are-now-fired-during-ime-composition/">keydown and keyup events are now fired during IME composition</a> ). 若要忽略复合事件中所有 <code>keydown</code> 事件, 可以按照如下代码修改 (229是某个在IME中触发的键盘事件对应的 <code>keyCode</code>):</p>
+
+<pre class="brush: js">eventTarget.addEventListener("keydown", event =&gt; {
+  if (event.isComposing || event.keyCode === 229) {
+    return;
+  }
+  // do something
+});
+</pre>
+
+<h2 id="示例">示例</h2>
+
+<h3 id="addEventListener_keydown_示例">addEventListener keydown 示例</h3>
+
+<p>这个例子展示了当你在{{HtmlElement("input")}}元素中按下一个按键时， {{domxref("KeyboardEvent.code")}} 的取值 </p>
+
+<pre class="brush: html">&lt;input placeholder="Click here, then press down a key." size="40"&gt;
+&lt;p id="log"&gt;&lt;/p&gt;</pre>
+
+<pre class="brush: js">const input = document.querySelector('input');
+const log = document.getElementById('log');
+
+input.addEventListener('keydown', logKey);
+
+function logKey(e) {
+  log.textContent += ` ${e.code}`;
+}</pre>
+
+<p>{{EmbedLiveSample("addEventListener_keydown_example")}}</p>
+
+<h3 id="onkeydown_示例">onkeydown 示例</h3>
+
+<pre class="brush: js">input.onkeydown = logKey;</pre>
+
+<h2 id="Specifications">Specifications</h2>
+
+<table class="standard-table">
+ <thead>
+  <tr>
+   <th scope="col">Specification</th>
+   <th scope="col">Status</th>
+  </tr>
+ </thead>
+ <tbody>
+  <tr>
+   <td>{{SpecName("UI Events", "#event-type-keydown")}}</td>
+   <td>{{Spec2("UI Events")}}</td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="Browser_compatibility">Browser compatibility</h2>
+
+<p>{{Compat("api.Element.keydown_event")}}</p>
+
+<h2 id="See_also">See also</h2>
+
+<ul>
+ <li><code><a href="/en-US/docs/Web/API/HTMLElement/input_event">input</a></code></li>
+ <li><code><a href="/en-US/docs/Web/API/Element/keypress_event">keypress</a></code></li>
+ <li><code><a href="/en-US/docs/Web/API/Element/keyup_event">keyup</a></code></li>
+ <li><a href="/en-US/docs/Web/API/Document/keydown_event">Document <code>keydown</code> event</a></li>
+</ul>