Math.round()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

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。

描述

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()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

js
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

js
// 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:

js
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

规范

Specification
ECMAScript® 2025 Language Specification
# sec-math.round

浏览器兼容性

Report problems with this compatibility data on GitHub
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Chrome
Edge
Firefox
Opera
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Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
round

Legend

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Full support
Full support

参见