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+---
+title: Math.PI
+slug: Web/JavaScript/Reference/Global_Objects/Math/PI
+translation_of: Web/JavaScript/Reference/Global_Objects/Math/PI
+---
+<div>{{JSRef("Global_Objects", "Math")}}</div>
+
+<h2 id="Summary" name="Summary">概述</h2>
+
+<p><code><strong>Math.PI</strong></code> 表示一个圆的周长与直径的比例，约为 3.14159：</p>
+
+<p><math><semantics><mrow><mstyle mathvariant="monospace"><mi>Math.PI</mi></mstyle><mo>=</mo><mi>π</mi><mo>≈</mo><mn>3.14159</mn></mrow><annotation encoding="TeX">\mathtt{\mi{Math.PI}} = \pi \approx 3.14159</annotation></semantics></math></p>
+
+<div>{{js_property_attributes(0,0,0)}}</div>
+
+<h2 id="Description" name="Description">描述</h2>
+
+<p>由于 <code>PI</code> 是 <code>Math</code> 的静态属性，所以应该像这样使用：<code>Math.PI</code>，而不是作为你创建的 <code>Math</code> 实例的属性（<code>Math</code> 不是构造函数）。</p>
+
+<h2 id="Examples" name="Examples">示例</h2>
+
+<h3 id="Example:_Using_PI" name="Example:_Using_PI">使用 <code>Math.PI</code></h3>
+
+<p>下面的函数使用 Math.PI 计算给定半径的圆周长：</p>
+
+<pre class="brush:js  language-js" style="padding: 1em 0px 1em 30px; font-size: 14px; white-space: normal; color: rgb(77, 78, 83);"><code class="language-js" style="direction: ltr; white-space: pre;"><span class="keyword token" style="color: rgb(0, 119, 170);">function</span> calculateCircumference <span class="punctuation token" style="color: rgb(153, 153, 153);">(</span>radius<span class="punctuation token" style="color: rgb(153, 153, 153);">)</span> <span class="punctuation token" style="color: rgb(153, 153, 153);">{</span>
+  <span class="keyword token" style="color: rgb(0, 119, 170);">return</span> <span class="number token" style="color: rgb(153, 0, 85);">2</span> <span class="operator token" style="background: rgba(255, 255, 255, 0.498039); color: rgb(166, 127, 89);">*</span> Math<span class="punctuation token" style="color: rgb(153, 153, 153);">.</span>PI <span class="operator token" style="background: rgba(255, 255, 255, 0.498039); color: rgb(166, 127, 89);">*</span> radius<span class="punctuation token" style="color: rgb(153, 153, 153);">;</span>
+<span class="punctuation token" style="color: rgb(153, 153, 153);">}</span>
+
+<span class="function token" style="color: rgb(221, 74, 104);">calculateCircumference<span class="punctuation token" style="color: rgb(153, 153, 153);">(</span></span><span class="number token" style="color: rgb(153, 0, 85);">1</span><span class="punctuation token" style="color: rgb(153, 153, 153);">)</span><span class="punctuation token" style="color: rgb(153, 153, 153);">;</span> <span class="comment token" style="color: rgb(112, 128, 144);"> // 6.283185307179586</span></code></pre>
+
+<h2 id="规范">规范</h2>
+
+<table class="standard-table">
+ <tbody>
+  <tr>
+   <th scope="col">规范版本</th>
+   <th scope="col">规范状态</th>
+   <th scope="col">注解</th>
+  </tr>
+  <tr>
+   <td>ECMAScript 1st Edition. Implemented in JavaScript 1.0</td>
+   <td>Standard</td>
+   <td>Initial definition.</td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES5.1', '#sec-15.8.1.6', 'Math.PI')}}</td>
+   <td>{{Spec2('ES5.1')}}</td>
+   <td> </td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES6', '#sec-math.pi', 'Math.PI')}}</td>
+   <td>{{Spec2('ES6')}}</td>
+   <td> </td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+<p>{{ CompatibilityTable() }}</p>
+
+<div id="compat-desktop">
+<table class="compat-table">
+ <tbody>
+  <tr>
+   <th>Feature</th>
+   <th>Chrome</th>
+   <th>Firefox (Gecko)</th>
+   <th>Internet Explorer</th>
+   <th>Opera</th>
+   <th>Safari</th>
+  </tr>
+  <tr>
+   <td>Basic support</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+  </tr>
+ </tbody>
+</table>
+</div>
+
+<div id="compat-mobile">
+<table class="compat-table">
+ <tbody>
+  <tr>
+   <th>Feature</th>
+   <th>Android</th>
+   <th>Chrome for Android</th>
+   <th>Firefox Mobile (Gecko)</th>
+   <th>IE Mobile</th>
+   <th>Opera Mobile</th>
+   <th>Safari Mobile</th>
+  </tr>
+  <tr>
+   <td>Basic support</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+   <td>{{ CompatVersionUnknown() }}</td>
+  </tr>
+ </tbody>
+</table>
+</div>
+
+<h2 id="See_also" name="See_also">相关链接</h2>
+
+<ul>
+ <li>The {{jsxref("Global_Objects/Math", "Math")}} object it belongs to.</li>
+</ul>