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---
title: parseFloat
slug: Web/JavaScript/Reference/Global_Objects/parseFloat
tags:
- parseFloat()
translation_of: Web/JavaScript/Reference/Global_Objects/parseFloat
---
<div>
<div>
<div>{{jsSidebar("Objects")}}</div>
</div>
</div>
<p><strong><code>parseFloat()</code> </strong>函数解析一个参数（必要时先转换为字符串）并返回一个浮点数。</p>
<div>{{EmbedInteractiveExample("pages/js/globalprops-parsefloat.html")}}</div>
<h2 id="语法">语法</h2>
<pre class="syntaxbox">parseFloat(<em>string)</em></pre>
<h3 id="参数">参数</h3>
<dl>
<dt><code>string</code></dt>
<dd>需要被解析成为浮点数的值。</dd>
</dl>
<h3 id="返回值">返回值</h3>
<dl>
<dd>给定值被解析成浮点数。如果给定值不能被转换成数值，则会返回 {{jsxref("NaN")}}。</dd>
</dl>
<h2 id="描述">描述</h2>
<p><code>parseFloat</code>是个全局函数,不属于任何对象。</p>
<ul>
<li>如果 <code>parseFloat</code> 在解析过程中遇到了正号（<code>+</code>）、负号（<code>-</code> U+002D HYPHEN-MINUS）、数字（<code>0</code>-<code>9</code>）、小数点（<code>.</code>）、或者科学记数法中的指数（e 或 E）以外的字符，则它会忽略该字符以及之后的所有字符，返回当前已经解析到的浮点数。</li>
<li>第二个小数点的出现也会使解析停止（在这之前的字符都会被解析）。</li>
<li>参数首位和末位的空白符会被忽略。</li>
<li>如果参数字符串的第一个字符不能被解析成为数字,<code>则</code> <code>parseFloat</code> 返回 <code>NaN</code>。</li>
<li><code>parseFloat</code> 也可以解析并返回 {{jsxref("Infinity")}}。</li>
<li><code>parseFloat</code>解析 {{jsxref("BigInt")}} 为 {{jsxref("Number", "Numbers")}}, 丢失精度。因为末位 <code>n</code> 字符被丢弃。</li>
</ul>
<p>考虑使用 {{jsxref("Number", "Number(<em>value</em>)")}} 进行更严谨的解析，只要参数带有无效字符就会被转换为 {{jsxref("NaN")}} 。</p>
<p><code>parseFloat</code> 也可以转换一个已经定义了 <code>toString</code> 或者 <code>valueOf</code> 方法的对象，它返回的值和在调用该方法的结果上调用 <code>parseFloat</code> 值相同。</p>
<h2 id="例子">例子</h2>
<h3 id="例子_parseFloat返回正常数字">例子: <code>parseFloat</code>返回正常数字</h3>
<p>下面的例子都返回<strong>3.14</strong></p>
<pre class="brush: js line-numbers language-js"><code class="language-js"><span class="function token">parseFloat</span><span class="punctuation token">(</span><span class="number token">3.14</span><span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="function token">parseFloat</span><span class="punctuation token">(</span><span class="string token">'3.14'</span><span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="function token">parseFloat</span><span class="punctuation token">(</span><span class="string token">' 3.14 '</span><span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="function token">parseFloat</span><span class="punctuation token">(</span><span class="string token">'314e-2'</span><span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="function token">parseFloat</span><span class="punctuation token">(</span><span class="string token">'0.0314E+2'</span><span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="function token">parseFloat</span><span class="punctuation token">(</span><span class="string token">'3.14some non-digit characters'</span><span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="function token">parseFloat</span><span class="punctuation token">(</span><span class="punctuation token">{</span> <span class="function function-variable token">toString</span><span class="punctuation token">:</span> <span class="keyword token">function</span><span class="punctuation token">(</span><span class="punctuation token">)</span> <span class="punctuation token">{</span> <span class="keyword token">return</span> <span class="string token">"3.14"</span> <span class="punctuation token">}</span> <span class="punctuation token">}</span><span class="punctuation token">)</span><span class="punctuation token">;</span></code></pre>
<h3 id="parseFloat返回NaN"><code>parseFloat</code>返回NaN</h3>
<p>下面的例子将返回<code>NaN</code></p>
<pre class="brush: js">parseFloat("FF2");
</pre>
<h3 id="parseFloat_和_BigInt">parseFloat 和 BigInt</h3>
<p>以下例子均返回 <code>900719925474099300</code>，当整数太大以至于不能被转换时将失去精度。</p>
<pre class="brush: js line-numbers language-js"><code class="language-js"><span class="function token">parseFloat</span><span class="punctuation token">(</span><span class="number token">900719925474099267n</span><span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="function token">parseFloat</span><span class="punctuation token">(</span><span class="string token">'900719925474099267n'</span><span class="punctuation token">)</span><span class="punctuation token">;</span></code></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>ECMAScript 1st Edition.</td>
<td>Standard</td>
<td>Initial definition.</td>
</tr>
<tr>
<td>{{SpecName('ES5.1', '#sec-15.1.2.3', 'parseFloat')}}</td>
<td>{{Spec2('ES5.1')}}</td>
<td></td>
</tr>
<tr>
<td>{{SpecName('ES6', '#sec-parsefloat-string', 'parseFloat')}}</td>
<td>{{Spec2('ES6')}}</td>
<td></td>
</tr>
<tr>
<td>{{SpecName('ESDraft', '#sec-parsefloat-string', 'parseFloat')}}</td>
<td>{{Spec2('ESDraft')}}</td>
<td></td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p>{{Compat("javascript.builtins.parseFloat")}}</p>
<h2 id="See_Also" name="See_Also">相关链接</h2>
<ul>
<li>{{jsxref("Global_Objects/parseInt", "parseInt()")}}</li>
<li>{{jsxref("Number.parseFloat()")}}</li>
<li>{{jsxref("Number.parseInt()")}}</li>
<li>{{jsxref("Global_Objects/isNaN", "isNaN()")}}</li>
</ul>
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