Javascript’s toFixed Implementation without Rounding

  • 时间：2020-09-24 11:54:15
  • 分类：网络文摘
  • 阅读：80 次
JS Javascript's toFixed Implementation without Rounding javascript

NodeJs / Javascript

In Javascript, you can use Number.prototype.toFixed(n) that will return a string version that has n decimal places. For example,

1
2

(3).toFixed(3) // "3.000"
(3.15).toFixed(3) // "3.150"
(3).toFixed(3) // "3.000"
(3.15).toFixed(3) // "3.150"

The toFixed will round up the last digit (from 6 not 5):

1
2

(3.1595).toFixed(3) // "3.159"
(3.1596).toFixed(3) // "3.160"
(3.1595).toFixed(3) // "3.159"
(3.1596).toFixed(3) // "3.160"

If we don’t want to round-up the decimal values, we can to this thanks to Regular Expression Matching:

1
2
3
4
5
6
7
8
9
10

Number.prototype.toFixedNoRounding = function(n) {
    const reg = new RegExp("^-?\\d+(?:\\.\\d{0," + n + "})?", "g")
    const a = this.toString().match(reg)[0];
    const dot = a.indexOf(".");
    if (dot === -1) { // integer, insert decimal dot and pad up zeros
        return a + "." + "0".repeat(n);
    }
    const b = n - (a.length - dot) + 1;
    return b > 0 ? (a + "0".repeat(b)) : a;
 }
Number.prototype.toFixedNoRounding = function(n) {
    const reg = new RegExp("^-?\\d+(?:\\.\\d{0," + n + "})?", "g")
    const a = this.toString().match(reg)[0];
    const dot = a.indexOf(".");
    if (dot === -1) { // integer, insert decimal dot and pad up zeros
        return a + "." + "0".repeat(n);
    }
    const b = n - (a.length - dot) + 1;
    return b > 0 ? (a + "0".repeat(b)) : a;
 }

The idea is to convert the number into string (via toString()) and use the RegExp to truncate the extra zeros (without rounding). Next, we find out the position of decimal dot (if any), and pad up the zeros for exactly n decimal places. We compute the number of zeros we need and if it is positive, we use the String.prototype.repeat() to insert additional zeros at the end. We could also use the String.prototype.padEnd() which may seem a bit less intuitive.

1
2
3
4
5
6
7

console.log((3.1999).toFixedNoRounding(3));  // 3.199
console.log((3.19923413412349).toFixedNoRounding(3)); // 3.199
console.log((3.01).toFixedNoRounding(3)); // 3.010
console.log((3.01).toFixedNoRounding(4)); // 3.0100
console.log((3.01).toFixedNoRounding(5)); // 3.01000
console.log((3).toFixedNoRounding(3)); // 3.000
console.log((-3).toFixedNoRounding(3)); // -3.000
console.log((3.1999).toFixedNoRounding(3));  // 3.199
console.log((3.19923413412349).toFixedNoRounding(3)); // 3.199
console.log((3.01).toFixedNoRounding(3)); // 3.010
console.log((3.01).toFixedNoRounding(4)); // 3.0100
console.log((3.01).toFixedNoRounding(5)); // 3.01000
console.log((3).toFixedNoRounding(3)); // 3.000
console.log((-3).toFixedNoRounding(3)); // -3.000

–EOF (The Ultimate Computing & Technology Blog) —

推荐阅读：
留一点休息给自己作文　 看图写话 补课 桓昕昱|小学作文　 家乡的如意湖作文　 三八妇女节珍贵的礼物作文　 中国魂，民族魂!　 我的娃打架？小打小闹？　 植树问题例题解答　 数学还原问题例题解答　 流水问题数量关系详解　 各种行程问题数量关系　
评论列表
添加评论