From 218934fa2ed1c702a6d3923d2aa2cc6b43c48684 Mon Sep 17 00:00:00 2001 From: Peter Bengtsson Date: Tue, 8 Dec 2020 14:43:23 -0500 Subject: initial commit --- .../reference/global_objects/math/round/index.html | 212 +++++++++++++++++++++ 1 file changed, 212 insertions(+) create mode 100644 files/zh-tw/web/javascript/reference/global_objects/math/round/index.html (limited to 'files/zh-tw/web/javascript/reference/global_objects/math/round') diff --git a/files/zh-tw/web/javascript/reference/global_objects/math/round/index.html b/files/zh-tw/web/javascript/reference/global_objects/math/round/index.html new file mode 100644 index 0000000000..5e6a0ea694 --- /dev/null +++ b/files/zh-tw/web/javascript/reference/global_objects/math/round/index.html @@ -0,0 +1,212 @@ +--- +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 +--- +
{{JSRef}}
+ +

Math.round() 函數回傳四捨五入後的近似值.

+ +

表達式

+ +
Math.round(x)
+ +

參數

+ +
+
x
+
數字.
+
+ +

描述

+ +

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.

+ +

由於 round() 是靜態的方法, 所以總是得這樣使用 Math.round(), 而非作為 Math 物件的一個方法 (Math並沒有建構子).

+ +

範例

+ +

使用 Math.round()

+ +
// 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;
+
+ +

十進位近似值

+ +
// 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
+
+ +

規範

+ + + + + + + + + + + + + + + + + + + + + + + + +
規範狀態註
{{SpecName('ES1')}}{{Spec2('ES1')}}Initial definition. Implemented in JavaScript 1.0.
{{SpecName('ES5.1', '#sec-15.8.2.15', 'Math.round')}}{{Spec2('ES5.1')}} 
{{SpecName('ES6', '#sec-math.round', 'Math.round')}}{{Spec2('ES6')}} 
+ +

瀏覽器相容性

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{{CompatibilityTable}}
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+ + + + + + + + + + + + + + + + + + + +
FeatureChromeFirefox (Gecko)Internet ExplorerOperaSafari (WebKit)
Basic support{{CompatVersionUnknown}}{{CompatVersionUnknown}}{{CompatVersionUnknown}}{{CompatVersionUnknown}}{{CompatVersionUnknown}}
+
+ +
+ + + + + + + + + + + + + + + + + + + +
FeatureAndroidFirefox Mobile (Gecko)IE PhoneOpera MobileSafari Mobile
Basic support{{CompatVersionUnknown}}{{CompatVersionUnknown}}{{CompatVersionUnknown}}{{CompatVersionUnknown}}{{CompatVersionUnknown}}
+
+ +

參見

+ + -- cgit v1.2.3-54-g00ecf