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+---
+title: Math.round()
+slug: Web/JavaScript/Reference/Global_Objects/Math/round
+tags:
+  - Math.round()
+  - 四舍五入
+translation_of: Web/JavaScript/Reference/Global_Objects/Math/round
+---
+<p>{{JSRef}}</p>
+
+<p><code><strong>Math.round()</strong></code> 函数返回一个数字四舍五入后最接近的整数。</p>
+
+<h2 id="Syntax" name="Syntax">语法</h2>
+
+<pre class="syntaxbox"><code>Math.round(<em>x</em>) </code></pre>
+
+<h3 id="Parameters" name="Parameters">参数</h3>
+
+<dl>
+ <dt><code>x</code></dt>
+ <dd>一个数值。</dd>
+ <dt>
+ <h3 id="返回值">返回值</h3>
+
+ <p>给定数字的值四舍五入到最接近的整数。</p>
+ </dt>
+</dl>
+
+<h2 id="Description" name="Description">描述</h2>
+
+<p>如果参数的小数部分大于 0.5，则舍入到相邻的绝对值更大的整数。 如果参数的小数部分小于 0.5，则舍入到相邻的绝对值更小的整数。如果参数的小数部分恰好等于0.5，则舍入到相邻的在正无穷（+∞）方向上的整数。<strong>注意，与很多其他语言中的<code>round()函数</code>不同，<code>Math.round()并不总是舍入到远离0的方向（尤其是在负数的小数部分恰好等于0.5的情况下）。</code></strong></p>
+
+<p>因为 <code>round()</code> 是 <code>Math</code> 的静态方法，你应该直接使用 <code>Math.round()</code>，而不是作为你创建的 <code>Math</code> 对象的一个实例方法来使用（<code>Math</code>没有构造函数）。</p>
+
+<h2 id="Examples" name="Examples">示例</h2>
+
+<pre class="brush:js">x = Math.round(20.49);   //20
+x = Math.round(20.5);    //21
+x = Math.round(-20.5);   //-20
+x = Math.round(-20.51);  //-21
+</pre>
+
+<h3 id="Example:_Decimal_rounding" name="Example:_Decimal_rounding" style="line-height: 24px;">小数舍入</h3>
+
+<pre class="brush: js">// 闭包
+(function(){
+
+  /**
+   * Decimal adjustment of a number.
+   *
+   * @param {String}  type  The type of adjustment.
+   * @param {Number}  value The number.
+   * @param {Integer} exp   The exponent (the 10 logarithm of the adjustment base).
+   * @returns {Number}      The adjusted value.
+   */
+  function decimalAdjust(type, value, exp) {
+    // If the exp is undefined or zero...
+    if (typeof exp === 'undefined' || +exp === 0) {
+      return Math[type](value);
+    }
+    value = +value;
+    exp = +exp;
+    // If the value is not a number or the exp is not an integer...
+    if (isNaN(value) || !(typeof exp === 'number' &amp;&amp; exp % 1 === 0)) {
+      return NaN;
+    }
+    // Shift
+    value = value.toString().split('e');
+    value = Math[type](+(value[0] + 'e' + (value[1] ? (+value[1] - exp) : -exp)));
+    // Shift back
+    value = value.toString().split('e');
+    return +(value[0] + 'e' + (value[1] ? (+value[1] + exp) : exp));
+  }
+
+  // Decimal round
+  if (!Math.round10) {
+    Math.round10 = function(value, exp) {
+      return decimalAdjust('round', value, exp);
+    };
+  }
+  // Decimal floor
+  if (!Math.floor10) {
+    Math.floor10 = function(value, exp) {
+      return decimalAdjust('floor', value, exp);
+    };
+  }
+  // Decimal ceil
+  if (!Math.ceil10) {
+    Math.ceil10 = function(value, exp) {
+      return decimalAdjust('ceil', value, exp);
+    };
+  }
+
+})();
+
+// Round
+Math.round10(55.55, -1); // 55.6
+Math.round10(55.549, -1); // 55.5
+Math.round10(55, 1); // 60
+Math.round10(54.9, 1); // 50
+Math.round10(-55.55, -1); // -55.5
+Math.round10(-55.551, -1); // -55.6
+Math.round10(-55, 1); // -50
+Math.round10(-55.1, 1); // -60
+Math.round10(1.005, -2); // 1.01 -- compare this with Math.round(1.005*100)/100 above
+// Floor
+Math.floor10(55.59, -1); // 55.5
+Math.floor10(59, 1); // 50
+Math.floor10(-55.51, -1); // -55.6
+Math.floor10(-51, 1); // -60
+// Ceil
+Math.ceil10(55.51, -1); // 55.6
+Math.ceil10(51, 1); // 60
+Math.ceil10(-55.59, -1); // -55.5
+Math.ceil10(-59, 1); // -50
+</pre>
+
+<p>或：</p>
+
+<pre class="brush: js">function round(number, precision) {
+    return Math.round(+number + 'e' + precision) / Math.pow(10, precision);
+    //same as:
+    //return Number(Math.round(+number + 'e' + precision) + 'e-' + precision);
+}
+
+round(1.005, 2);    //1.01
+</pre>
+
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+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1083px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1102px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1121px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1140px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1159px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1178px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1197px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1216px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1235px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1254px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1273px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1292px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1311px; background: 0px 0px;"> </div>
+
+<div class="line-number" style="margin-top: 1em; position: absolute; left: 0px; right: 0px; line-height: inherit; top: 1330px; background: 0px 0px;"> </div>
+
+<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-15.8.2.15', 'Math.round')}}</td>
+   <td>{{Spec2('ES5.1')}}</td>
+   <td> </td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES6', '#sec-math.round', 'Math.round')}}</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>
+
+<div>{{CompatibilityTable}}</div>
+
+<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 (WebKit)</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;">Firefox Mobile (Gecko)</th>
+   <th style="line-height: 16px;">IE Phone</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>
+  </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>{{jsxref("Math.abs()")}}</li>
+ <li>{{jsxref("Math.ceil()")}}</li>
+ <li>{{jsxref("Math.floor()")}}</li>
+</ul>