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119 lines
7.1 KiB
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<article id="wikiArticle">
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<p id="Summary"></p><p></p>
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<h2 id="Summary" name="Summary">概述</h2>
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<p><code><strong>Math.log()</strong></code> 函数返回一个数的自然对数，即：</p>
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<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>
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<h2 id="Syntax" name="Syntax">语法</h2>
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<pre><code class="language-javascript"><code>Math.log(<em>x</em>)</code></code></pre>
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<h3 id="Parameters" name="Parameters">参数</h3>
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<dl>
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<dt><code>x</code></dt>
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<dd>一个数字.</dd>
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</dl>
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<h2 id="Description" name="Description">描述</h2>
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<p>如果指定的 <code>number</code> 为负数，则返回值为 <code>NaN</code>。</p>
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<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>
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<h2 id="示例">示例</h2>
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<h3 id="Example:_Using_Math.log" name="Example:_Using_Math.log">例子1:使用<code>Math.log</code></h3>
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<p>下面的函数返回指定变量的自然对数：</p>
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<pre><code class="language-js">Math.log(-1); // NaN, out of range
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Math.log(0); // -Infinity
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Math.log(1); // 0
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Math.log(10); // 2.302585092994046</code></pre>
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<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>
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<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>
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<pre><code class="language-js">function getBaseLog(x, y) {
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return Math.log(y) / Math.log(x);
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}</code></pre>
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<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>
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<h2 id="规范" style="margin-bottom: 20px; line-height: 30px;">规范</h2>
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<table class="standard-table">
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<tbody>
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<tr>
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<th scope="col">规范版本</th>
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<th scope="col">规范状态</th>
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<th scope="col">注解</th>
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</tr>
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<tr>
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<td>ECMAScript 1st Edition. Implemented in JavaScript 1.0</td>
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<td>Standard</td>
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<td>Initial definition.</td>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.8.2.10" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">Math.log</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td> </td>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-math.log" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Math.log</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td> </td>
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</tr>
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</tbody>
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</table>
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<h2 id="浏览器兼容性" style="margin-bottom: 20px; line-height: 30px;">浏览器兼容性</h2>
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<p></p><div class="blockIndicator warning"><strong><a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">We're converting our compatibility data into a machine-readable JSON format</a></strong>.
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This compatibility table still uses the old format,
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because we haven't yet converted the data it contains.
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<strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
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<div class="htab">
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<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
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<ul>
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<li class="selected"><a>Desktop</a></li>
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<li><a>Mobile</a></li>
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</ul>
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</div><p></p>
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<div id="compat-desktop">
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<table class="compat-table">
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<tbody>
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<tr>
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<th style="line-height: 16px;">Feature</th>
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<th style="line-height: 16px;">Chrome</th>
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<th style="line-height: 16px;">Firefox (Gecko)</th>
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<th style="line-height: 16px;">Internet Explorer</th>
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<th style="line-height: 16px;">Opera</th>
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<th style="line-height: 16px;">Safari</th>
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</tr>
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<tr>
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<td>Basic support</td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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</tr>
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</tbody>
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</table>
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</div>
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<div id="compat-mobile">
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<table class="compat-table">
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<tbody>
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<tr>
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<th style="line-height: 16px;">Feature</th>
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<th style="line-height: 16px;">Android</th>
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<th style="line-height: 16px;">Chrome for Android</th>
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<th style="line-height: 16px;">Firefox Mobile (Gecko)</th>
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<th style="line-height: 16px;">IE Mobile</th>
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<th style="line-height: 16px;">Opera Mobile</th>
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<th style="line-height: 16px;">Safari Mobile</th>
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</tr>
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<tr>
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<td>Basic support</td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
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</tr>
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</tbody>
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</table>
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</div>
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<h2 id="See_also" name="See_also" style="margin-bottom: 20px; line-height: 30px;">相关链接</h2>
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<p></p>
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<ul>
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<li>The <a href="Reference/Global_Objects/Math" title="Math 是一个内置对象， 它具有数学常数和函数的属性和方法。不是一个函数对象。"><code>Math</code></a> object it belongs to.</li>
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<li><a href="Reference/Global_Objects/Math/exp" title="Math.exp() 函数返回 ex，x 表示参数，e 是欧拉常数（Euler's constant），自然对数的底数。"><code>Math.exp()</code></a></li>
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<li><a href="Reference/Global_Objects/Math/pow" title="Math.pow() 函数返回基数（base）的指数（exponent）次幂，即 baseexponent。"><code>Math.pow()</code></a></li>
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</ul>
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</article> |