---
title: Math.log2()
slug: Web/JavaScript/Reference/Global_Objects/Math/log2
translation_of: Web/JavaScript/Reference/Global_Objects/Math/log2
---
<div>
<div>{{JSRef("Global_Objects", "Math")}}</div>
</div>

<h2 id="Summary" name="Summary">概述</h2>

<p><code><strong>Math.log2()</strong></code> 函数返回一个数字以 2 为底的对数.</p>

<h2 id="Syntax" name="Syntax">语法</h2>

<pre class="syntaxbox"><code>Math.log2(<em>x</em>)</code></pre>

<h3 id="Parameters" name="Parameters">参数</h3>

<dl>
 <dt><code>x</code></dt>
 <dd>任意数字.</dd>
</dl>

<h2 id="Description" name="Description">描述</h2>

<p>如果传入的参数小于 0, 则返回 <code>NaN</code>.</p>

<h2 id="Examples" name="Examples">示例</h2>

<pre class="brush:js">Math.log2(2)     // 1
Math.log2(1024)  // 10
Math.log2(1)     // 0
Math.log2(0)     // -Infinity
Math.log2(-2)    // NaN
Math.log2("1024")// 10
Math.log2("foo") // NaN
</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-math.log2', 'Math.log2')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>Initial definition.</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>{{CompatNo}}</td>
   <td>{{CompatGeckoDesktop("25")}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</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>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatGeckoMobile("25")}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
  </tr>
 </tbody>
</table>
</div>

<h2 id="See_also" name="See_also">相关链接</h2>

<ul>
 <li>{{jsxref("Global_Objects/Math", "Math")}} 对象.</li>
</ul>