Javascript’s toFixed Implementation without Rounding

  • 时间：2020-09-24 11:54:15
  • 分类：网络文摘
  • 阅读：118 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,

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(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):

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(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:

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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.

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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) —

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