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---
title: Math.log()
slug: Web/JavaScript/Reference/Global_Objects/Math/log
translation_of: Web/JavaScript/Reference/Global_Objects/Math/log
---
<p id="Summary">{{JSRef("Global_Objects", "Math")}}</p>
<h2 id="Summary" name="Summary">概述</h2>
<p><code><strong>Math.log()</strong></code> 函数返回一个数的自然对数，即：</p>
<p><math><semantics><mrow><mo>∀</mo><mi>x</mi><mo>></mo><mn>0</mn><mo>,</mo><mstyle mathvariant="monospace"><mrow><mo lspace="0em" rspace="thinmathspace">Math.log</mo><mo stretchy="false">(</mo><mi>x</mi><mo stretchy="false">)</mo></mrow></mstyle><mo>=</mo><mo lspace="0em" rspace="0em">ln</mo><mo stretchy="false">(</mo><mi>x</mi><mo stretchy="false">)</mo><mo>=</mo><mtext>the unique</mtext><mspace width="thickmathspace"></mspace><mi>y</mi><mspace width="thickmathspace"></mspace><mtext>such that</mtext><mspace width="thickmathspace"></mspace><msup><mi>e</mi><mi>y</mi></msup><mo>=</mo><mi>x</mi></mrow><annotation encoding="TeX">\forall x > 0, \mathtt{\operatorname{Math.log}(x)} = \ln(x) = \text{the unique} \; y \; \text{such that} \; e^y = x</annotation></semantics></math></p>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre class="syntaxbox"><code>Math.log(<em>x</em>)</code></pre>
<h3 id="Parameters" name="Parameters">参数</h3>
<dl>
<dt><code>x</code></dt>
<dd>一个数字.</dd>
</dl>
<h2 id="Description" name="Description">描述</h2>
<p>如果指定的 <code>number</code> 为负数，则返回值为 <code>NaN</code>。</p>
<p>由于 <code style="font-style: normal;">log</code> 是 <code style="font-style: normal;">Math</code> 的静态方法，所以应该像这样使用：<code style="font-style: normal;">Math.log()</code>，而不是作为你创建的 <code style="font-style: normal;">Math</code> 对象的方法。</p>
<h2 id="示例">示例</h2>
<h3 id="Example:_Using_Math.log" name="Example:_Using_Math.log">例子1:使用<code>Math.log</code></h3>
<p>下面的函数返回指定变量的自然对数：</p>
<pre class="brush:js">Math.log(-1); // NaN, out of range
Math.log(0); // -Infinity
Math.log(1); // 0
Math.log(10); // 2.302585092994046</pre>
<h3 id="例子2_使用Math.log时基于不同的底数">例子2: 使用<code>Math.log</code>时基于不同的底数</h3>
<p>下面的函数返回以 <code>x</code> 为底 <code>y</code> 的对数（即log<sub>x</sub> y）：</p>
<pre class="brush:js">function getBaseLog(x, y) {
return Math.log(y) / Math.log(x);
}</pre>
<p>如果你运行 <code>getBaseLog(10, 1000)，则会返回</code><code>2.9999999999999996，非常接近实际答案：3，原因是浮点数精度问题。</code></p>
<h2 id="规范" style="margin-bottom: 20px; line-height: 30px;">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-15.8.2.10', 'Math.log')}}</td>
<td>{{Spec2('ES5.1')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES6', '#sec-math.log', 'Math.log')}}</td>
<td>{{Spec2('ES6')}}</td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性" style="margin-bottom: 20px; line-height: 30px;">浏览器兼容性</h2>
<p>{{ CompatibilityTable() }}</p>
<div id="compat-desktop">
<table class="compat-table">
<tbody>
<tr>
<th style="line-height: 16px;">Feature</th>
<th style="line-height: 16px;">Chrome</th>
<th style="line-height: 16px;">Firefox (Gecko)</th>
<th style="line-height: 16px;">Internet Explorer</th>
<th style="line-height: 16px;">Opera</th>
<th style="line-height: 16px;">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 style="line-height: 16px;">Feature</th>
<th style="line-height: 16px;">Android</th>
<th style="line-height: 16px;">Chrome for Android</th>
<th style="line-height: 16px;">Firefox Mobile (Gecko)</th>
<th style="line-height: 16px;">IE Mobile</th>
<th style="line-height: 16px;">Opera Mobile</th>
<th style="line-height: 16px;">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" style="margin-bottom: 20px; line-height: 30px;">相关链接</h2>
<p></p>
<ul>
<li>The {{jsxref("Global_Objects/Math", "Math")}} object it belongs to.</li>
<li>{{jsxref("Math.exp()")}}</li>
<li>{{jsxref("Math.pow()")}}</li>
</ul>
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