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

<h2 id="Summary">概述</h2>

<p><code><strong>toPrecision()</strong></code> 方法以指定的精度返回该数值对象的字符串表示。</p>

<h2 id="Syntax">语法</h2>

<pre class="syntaxbox"><code><em>numObj</em>.toPrecision(<em>precision</em>)</code></pre>

<h3 id="Parameter">参数</h3>

<dl>
 <dt>precision</dt>
 <dd>可选。一个用来指定有效数个数的整数。</dd>
</dl>

<h3 id="Returns">返回值</h3>

<p>以定点表示法或指数表示法表示的一个数值对象的字符串表示，四舍五入到 <code>precision</code> 参数指定的显示数字位数。查看 {{jsxref("Number.prototype.toFixed()")}} 方法关于四舍五入的讨论，同样应用于 <code>toPrecision</code> 方法。</p>

<p>如果忽略 <code>precision</code> 参数，则该方法表现类似于 {{jsxref("Number.prototype.toString()")}}。如果该参数是一个非整数值，将会向下舍入到最接近的整数。</p>

<h3 id="异常">异常</h3>

<dl>
 <dt>{{jsxref("Global_Objects/RangeError", "RangeError")}}</dt>
 <dd>如果 <em>precison</em> 参数不在 1 和 100 （包括）之间，将会抛出一个 <code>RangeError</code> 。执行环境也可以支持更大或更小的范围。ECMA-262 只需要最多 21 位显示数字。</dd>
</dl>

<h2 id="Example">示例</h2>

<pre class="brush: js">var numObj = 5.123456;
console.log("numObj.toPrecision()  is " + numObj.toPrecision());  //输出 5.123456
console.log("numObj.toPrecision(5) is " + numObj.toPrecision(5)); //输出 5.1235
console.log("numObj.toPrecision(2) is " + numObj.toPrecision(2)); //输出 5.1
console.log("numObj.toPrecision(1) is " + numObj.toPrecision(1)); //输出 5

// 注意：在某些情况下会以指数表示法返回
console.log((1234.5).toPrecision(2)); // "1.2e+3"
</pre>

<h2 id="规范">规范</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">规范</th>
   <th scope="col">规范状态</th>
   <th scope="col">注解</th>
  </tr>
  <tr>
   <td>ECMAScript 3rd Edition. Implemented in JavaScript 1.5</td>
   <td>Standard</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-15.7.4.7', 'Number.prototype.toPrecision')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-number.prototype.toprecision', 'Number.prototype.toPrecision')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td> </td>
  </tr>
 </tbody>
</table>

<h2 id="浏览器兼容性">浏览器兼容性</h2>

<p>{{ CompatibilityTable() }}</p>

<div>
<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>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
  </tr>
 </tbody>
</table>
</div>

<div>
<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>{{ 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">相关链接</h2>

<ul>
 <li>{{jsxref("Number.prototype.toFixed()")}}</li>
 <li>{{jsxref("Number.prototype.toExponential()")}}</li>
 <li>{{jsxref("Number.prototype.toString()")}}</li>
</ul>