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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>&gt;</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 &gt; 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>