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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' &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>
+
+<h2 id="規範">規範</h2>
+
+<table class="standard-table">
+ <tbody>
+  <tr>
+   <th scope="col">規範</th>
+   <th scope="col">狀態</th>
+   <th scope="col">註</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-15.8.2.15', '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="瀏覽器相容性">瀏覽器相容性</h2>
+
+<div>{{CompatibilityTable}}</div>
+
+<div id="compat-desktop">
+<table class="compat-table">
+ <tbody>
+  <tr>
+   <th>Feature</th>
+   <th>Chrome</th>
+   <th>Firefox (Gecko)</th>
+   <th>Internet Explorer</th>
+   <th>Opera</th>
+   <th>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>Feature</th>
+   <th>Android</th>
+   <th>Firefox Mobile (Gecko)</th>
+   <th>IE Phone</th>
+   <th>Opera Mobile</th>
+   <th>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="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>