---
title: Math.floor()
slug: Web/JavaScript/Reference/Global_Objects/Math/floor
tags:
  - Math.floor()
  - 向下取整
translation_of: Web/JavaScript/Reference/Global_Objects/Math/floor
---
<div>{{JSRef}}<br>
 </div>

<p><code><strong>Math.floor()</strong></code> 返回小于或等于一个给定数字的最大整数。</p>

<div class="note">
<p><strong>备注：</strong>可以理解 <code><strong>Math.floor()</strong></code>为向下取整</p>
</div>

<h2 id="Syntax">语法</h2>

<pre class="syntaxbox"><code>Math.floor(<em>x</em>) </code></pre>

<h3 id="Parameters">参数</h3>

<dl>
 <dt><code>x</code></dt>
 <dd>一个数字。</dd>
</dl>

<h3 id="返回值">返回值 </h3>

<p>一个表示小于或等于指定数字的最大整数的数字。</p>

<h2 id="Description">描述</h2>

<p>由于 <code>floor</code> 是 <code>Math</code> 的一个静态方法，你总是应该像这样使用它 <code>Math.floor()</code>，而不是作为你创建的一个Math对象的一种方法（Math不是一个构造函数）。</p>

<h2 id="Examples">示例</h2>

<h3 id="Example_Using_Math.floor">例子：使用 <code>Math.floor</code></h3>

<pre class="brush:js">Math.floor( 45.95);
// 45
Math.floor( 45.05);
// 45
Math.floor( 4 );
// 4
Math.floor(-45.05);
// -46
Math.floor(-45.95);
// -46

</pre>

<h3 id="Example_Decimal_adjustment">例子：十进制调整</h3>

<pre class="brush:js">// Closure
(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
// 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>

<h2 id="规范">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-15.8.2.9', 'Math.floor')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td></td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-math.floor', 'Math.floor')}} </td>
   <td>{{Spec2('ES6')}}</td>
   <td></td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-math.floor', 'Math.floor')}}</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>

{{Compat}}

<h2 id="See_also">相关链接</h2>

<ul>
 <li>The {{jsxref("Global_Objects/Math", "Math")}} object it belongs to.</li>
 <li>{{jsxref("Math.abs")}}</li>
 <li>{{jsxref("Math.ceil()")}}</li>
 <li>{{jsxref("Math.round()")}}</li>
 <li>{{jsxref("Math.sign()")}}</li>
 <li>{{jsxref("Math.trunc()")}}</li>
</ul>