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---
title: Math.round()
slug: Web/JavaScript/Reference/Global_Objects/Math/round
tags:
- JavaScript
- Math
- Method
- Reference
translation_of: Web/JavaScript/Reference/Global_Objects/Math/round
---
<div>{{JSRef}}</div>
<p><strong><code>Math.round()</code></strong> 函數回傳四捨五入後的近似值.</p>
<h2 id="表達式">表達式</h2>
<pre class="syntaxbox"><code>Math.round(<var>x</var>)</code></pre>
<h3 id="參數">參數</h3>
<dl>
<dt><code>x</code></dt>
<dd>數字.</dd>
</dl>
<h2 id="描述">描述</h2>
<p>如果小數位的部分值大於 0.5, 這個值將會進位. 如果小數位的部分值小於 0.5, 這個值將不會進位.</p>
<p>由於 <code>round()</code> 是靜態的方法, 所以總是得這樣使用 <code>Math.round()</code>, 而非作為 <code>Math</code> 物件的一個方法 (<code>Math</code>並沒有建構子).</p>
<h2 id="範例">範例</h2>
<h3 id="使用_Math.round()">使用 <code>Math.round()</code></h3>
<pre class="brush: js">// Returns the value 20
x = Math.round(20.49);
// Returns the value 21
x = Math.round(20.5);
// Returns the value -20
x = Math.round(-20.5);
// Returns the value -21
x = Math.round(-20.51);
// Returns the value 1 (!)
// Note the rounding error because of inaccurate floating point arithmetics
// Compare this with Math.round10(1.005, -2) from the example below
x = Math.round(1.005*100)/100;
</pre>
<h3 id="十進位近似值">十進位近似值</h3>
<pre class="brush: js">// 閉包含數
(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' && 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>
<h2 id="規範">規範</h2>
{{Specifications}}
<h2 id="瀏覽器相容性">瀏覽器相容性</h2>
{{Compat}}
<h2 id="參見">參見</h2>
<ul>
<li>{{jsxref("Math.abs()")}}</li>
<li>{{jsxref("Math.ceil()")}}</li>
<li>{{jsxref("Math.floor()")}}</li>
<li>{{jsxref("Math.sign()")}} {{experimental_inline}}</li>
<li>{{jsxref("Math.trunc()")}} {{experimental_inline}}</li>
</ul>
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