uTools-Manuals/docs/javascript/Reference/Global_Objects/Math/round.html

<article id="wikiArticle">
<p></p><p></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><code class="language-javascript"><code>Math.round(<em>x</em>) </code></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><code class="language-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
</code></pre>
<h3 id="Example:_Decimal_rounding" name="Example:_Decimal_rounding" style="line-height: 24px;">小数舍入</h3>
<pre><code  class="language-javascript">// 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
</code></pre>
<p>或：</p>
<pre><code  class="language-javascript">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
</code></pre>
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<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><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.8.2.15" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">Math.round</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-math.round" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Math.round</small></a></td>
<td><span class="spec-Standard">Standard</span></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><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>.
            This compatibility table still uses the old format,
            because we haven't yet converted the data it contains.
            <strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
<div class="htab">
<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
<ul>
<li class="selected"><a>Desktop</a></li>
<li><a>Mobile</a></li>
</ul>
</div></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><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></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><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></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><a href="Reference/Global_Objects/Math/abs" title="Math.abs(x) 函数返回指定数字 “x“ 的绝对值。如下："><code>Math.abs()</code></a></li>
<li><a href="Reference/Global_Objects/Math/ceil" title="Math.ceil() 函数返回大于或等于一个给定数字的最小整数。"><code>Math.ceil()</code></a></li>
<li><a href="Reference/Global_Objects/Math/floor" title="Math.floor() 返回小于或等于一个给定数字的最大整数。"><code>Math.floor()</code></a></li>
</ul>
</article>