Javascript’s toFixed Implementation without Rounding

  • 时间：2020-09-24 11:54:15
  • 分类：网络文摘
  • 阅读：120 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：
春节作文300字400字500字600字800字　 加权平均数是什么意思？　 四舍五入法怎么用　 多边形的内角和公式　 梯形上底的定义是什么　 用放大镜看角变大了吗　 四位一级和三位分节　 求近似数的方法有哪几种　 自然数的产生　 有没有平方十米　
评论列表
添加评论