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+---
+title: Window.screenLeft
+slug: Web/API/Window/screenLeft
+translation_of: Web/API/Window/screenLeft
+---
+<div>{{APIRef}}</div>
+
+<p><code><strong>Window.screenLeft</strong></code> 是一个只读属性，它返回浏览器左边框到左边屏幕边缘的 CSS 像素数。</p>
+
+<div class="blockIndicator note">
+<p><strong>注意</strong>：<code>screenLeft</code> 只是 {{domxref("Window.screenX")}}  属性的别名，最初只被 IE 浏览器所支持。现在主流的浏览器都已支持该属性。</p>
+</div>
+
+<h2 id="Syntax" name="Syntax">语法</h2>
+
+<pre class="syntaxbox"><em>leftWindowPos</em> = window.screenLeft
+</pre>
+
+<h3 id="返回值">返回值</h3>
+
+<p>返回浏览器窗口到屏幕左边缘的 CSS 像素距离数值。</p>
+
+<h2 id="Specifications" name="Specifications">例子</h2>
+
+<p>在 <a href="https://mdn.github.io/dom-examples/screenleft-screentop/">screenleft-screentop</a> 项目中，展示了一个监听浏览器窗口位置变化，动态更新 canvas 的例子。在这个例子中，当你移动浏览器窗口位置，绘制在 canvas 上的圆也会对应移动。我们通过监听 <code>screenLeft</code>/<code>screenTop</code> 的变化，并使用 {{domxref("Window.requestAnimationFrame()")}} 来对 canvas 实时重绘，保证了浏览器窗口移动时，canvas 内部圆的位置也会发生对应的移动。</p>
+
+<pre class="brush: js">initialLeft = window.screenLeft + canvasElem.offsetLeft;
+initialTop = window.screenTop + canvasElem.offsetTop;
+
+function positionElem() {
+  let newLeft = window.screenLeft + canvasElem.offsetLeft;
+  let newTop = window.screenTop + canvasElem.offsetTop;
+
+  let leftUpdate = initialLeft - newLeft;
+  let topUpdate = initialTop - newTop;
+
+  ctx.fillStyle = 'rgb(0, 0, 0)';
+  ctx.fillRect(0, 0, width, height);
+  ctx.fillStyle = 'rgb(0, 0, 255)';
+  ctx.beginPath();
+  ctx.arc(leftUpdate + (width/2), topUpdate + (height/2) + 35, 50, degToRad(0), degToRad(360), false);
+  ctx.fill();
+
+  pElem.textContent = 'Window.screenLeft: ' + window.screenLeft + ', Window.screenTop: ' + window.screenTop;
+
+  window.requestAnimationFrame(positionElem);
+}
+
+window.requestAnimationFrame(positionElem);</pre>
+
+<p>如果浏览器不支持 <code>screenLeft</code>属性，我们还在代码中使用了一个 polyfill 来保证演示效果。通过设置 {{domxref("Window.screenX")}}/{{domxref("Window.screenY")}} 为对应别名来实现一样的功能。</p>
+
+<pre class="brush: js">if(!window.screenLeft) {
+  window.screenLeft = window.screenX;
+  window.screenTop = window.screenY;
+}</pre>
+
+<h2 id="Specifications" name="Specifications">说明</h2>
+
+<table class="standard-table" style="height: 49px; width: 1000px;">
+ <thead>
+  <tr>
+   <th scope="col">规范</th>
+   <th scope="col">状态</th>
+   <th scope="col">提交</th>
+  </tr>
+ </thead>
+ <tbody>
+  <tr>
+   <td>{{ SpecName('CSSOM View', '#dom-window-screenx', 'Window.screenLeft') }}</td>
+   <td>{{ Spec2('CSSOM View') }}</td>
+   <td>Initial definition.</td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+
+
+<p>{{Compat("api.Window.screenLeft")}}</p>
+
+<h2 id="参考">参考</h2>
+
+<ul>
+ <li>{{domxref("window.screenTop")}}</li>
+ <li>{{domxref("Window.screenX")}}</li>
+</ul>