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

<p><strong><code>Math.floor()</code></strong> 函式會回傳小於等於所給數字的最大整數。</p>

<div>{{EmbedInteractiveExample("pages/js/math-floor.html")}}</div>



<h2 id="語法">語法</h2>

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

<h3 id="參數">參數</h3>

<dl>
 <dt><code>x</code></dt>
 <dd>數字型態。</dd>
</dl>

<h3 id="回傳值">回傳值</h3>

<p>小於等於所給數字的最大整數。</p>

<h2 id="描述">描述</h2>

<p> <code>floor()</code> 是 <code>Math</code>的靜態函式, 所以不需實體化<code>Math</code> 物件, 只要直接呼叫 <code>Math.floor()</code>就能使用。</p>

<p>(此外<code>Math</code> 並不是建構子).</p>

<h2 id="範例">範例</h2>

<h3 id="使用_Math.floor">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="Decimal_adjustment">Decimal adjustment</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="規範">規範</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>{{SpecName('ES1')}}</td>
   <td>{{Spec2('ES1')}}</td>
   <td>Initial definition. Implemented in JavaScript 1.0.</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="瀏覽器相容性">瀏覽器相容性</h2>

<p>{{Compat("javascript.builtins.Math.floor")}}</p>

<h2 id="參見">參見</h2>

<ul>
 <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>