From 33058f2b292b3a581333bdfb21b8f671898c5060 Mon Sep 17 00:00:00 2001 From: Peter Bengtsson Date: Tue, 8 Dec 2020 14:40:17 -0500 Subject: initial commit --- .../reference/global_objects/math/round/index.html | 356 +++++++++++++++++++++ 1 file changed, 356 insertions(+) create mode 100644 files/zh-cn/web/javascript/reference/global_objects/math/round/index.html (limited to 'files/zh-cn/web/javascript/reference/global_objects/math/round/index.html') diff --git a/files/zh-cn/web/javascript/reference/global_objects/math/round/index.html b/files/zh-cn/web/javascript/reference/global_objects/math/round/index.html new file mode 100644 index 0000000000..c1fae674cc --- /dev/null +++ b/files/zh-cn/web/javascript/reference/global_objects/math/round/index.html @@ -0,0 +1,356 @@ +--- +title: Math.round() +slug: Web/JavaScript/Reference/Global_Objects/Math/round +tags: + - Math.round() + - 四舍五入 +translation_of: Web/JavaScript/Reference/Global_Objects/Math/round +--- +

{{JSRef}}

+ +

Math.round() 函数返回一个数字四舍五入后最接近的整数。

+ +

语法

+ +
Math.round(x)
+ +

参数

+ +
+
x
+
一个数值。
+
+

返回值

+ +

给定数字的值四舍五入到最接近的整数。

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+ +

描述

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如果参数的小数部分大于 0.5，则舍入到相邻的绝对值更大的整数。 如果参数的小数部分小于 0.5，则舍入到相邻的绝对值更小的整数。如果参数的小数部分恰好等于0.5，则舍入到相邻的在正无穷（+∞）方向上的整数。注意，与很多其他语言中的round()函数不同，Math.round()并不总是舍入到远离0的方向（尤其是在负数的小数部分恰好等于0.5的情况下）。

+ +

因为 round() 是 Math 的静态方法，你应该直接使用 Math.round()，而不是作为你创建的 Math 对象的一个实例方法来使用（Math没有构造函数）。

+ +

示例

+ +
x = Math.round(20.49);   //20
+x = Math.round(20.5);    //21
+x = Math.round(-20.5);   //-20
+x = Math.round(-20.51);  //-21
+
+ +

小数舍入

+ +
// 闭包
+(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
+
+ +

或：

+ +
function round(number, precision) {
+    return Math.round(+number + 'e' + precision) / Math.pow(10, precision);
+    //same as:
+    //return Number(Math.round(+number + 'e' + precision) + 'e-' + precision);
+}
+
+round(1.005, 2);    //1.01
+
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规范

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规范版本规范状态注解
ECMAScript 1st Edition. Implemented in JavaScript 1.0.StandardInitial definition.
{{SpecName('ES5.1', '#sec-15.8.2.15', 'Math.round')}}{{Spec2('ES5.1')}} 
{{SpecName('ES6', '#sec-math.round', 'Math.round')}}{{Spec2('ES6')}} 
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浏览器兼容性

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

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