可靠的JS舍入数字与3个十进制数的toFixed（2）

问题描述

我只想整理 1.275.toFixed（2），我期待 1.28 的回报，而不是1.27。

I am simply trying to round up 1.275.toFixed(2) and I was expecting a return of 1.28, rather than 1.27.

使用各种计算器和舍入​​到最接近的百分之一的简单方法，如果最后一位数大于或等于五，则应向上舍入。

Using various calculators and the simple method of rounding to the nearest hundredth, if the last digit is greater than or equal to five, it should round up.

如果这不能与toFixed（2）一起使用，它会如何？

If this doesn't work with toFixed(2), how would it?

人们询问console.log（1.275.toFixed（2））是否打印掉1.28，这是一个快速截图MacOS Chrome版本55.0.2883.95（64位）

People asking whether console.log(1.275.toFixed(2)) prints off 1.28, here's a quick screenshot MacOS Chrome Version 55.0.2883.95 (64-bit)

推荐答案

toFixed（） 方法在舍入中是不可靠的（请参阅，了解为何会出现这种情况）。

The toFixed() method is unreliable in its rounding (see Álvaro González' answer as to why this is the case).

在当前的Chrome和Firefox中，调用 toFixed（）会产生以下不一致的结果：

In both current Chrome and Firefox, calling toFixed() yields the following inconsistent results:

35.655.toFixed(2) // Yields "36.66" (correct)
35.855.toFixed(2) // Yields "35.85" (wrong, should be "35.86")

MDN描述了：

// Closure
(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

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