Math.floor() should take a second param to specify decimal place like _.floor

[JackieCalapristi]

I was completely floored when I found out that the current implementation of Math.floor() doesn't allow for specifying decimal place like _.floor() in lodash does. It would be nice to be able to pass in a second param to Math.floor() to specify decimal place for a little more precision.

Example

Math.floor(.046);
// => 0

vs.

_.floor(0.046, 2);
// => 0.04

[micnic]

I would propose to add this parameter to Math.trunc() instead.

[zloirock]

Adding additional arguments to existing methods like those will not work because of existing use cases like array.map(Math.floor).

[micnic]

@zloirock, agree, then the best option would be to introduce another method, like Math.toFixed(value, decimals) that would return a number value, naming can be different, not to be confused with Number.prototype.toFixed() that returns a string.

[Rudxain]

That 2nd param reminds me of Python's round. The only issue I see is that it assumes a constant base of 10, excluding all other bases, so a 3rd base/radix param should be added to make it future-proof, and more versatile in the present. A reason to support different bases is that both Number.prototype.toString and parseInt support it

[Andrew-Cottrell]

FYI, similar library functions in https://github.com/stdlib-js/stdlib

• @stdlib/math/base/special/ceiln — Round a numeric value to the nearest multiple of 10^n toward positive infinity.
• @stdlib/math/base/special/floorn — Round a numeric value to the nearest multiple of 10^n toward negative infinity.
• @stdlib/math/base/special/roundn — Round a numeric value to the nearest multiple of 10^n.
• @stdlib/math/base/special/truncn — Round a numeric value to the nearest multiple of 10^n toward zero.

---

I also implemented a similar function in my library

/**
 * Rounds a number to the nearest specified power of ten.
 * @param {number} number - The number to round.
 * @param {number} powerOfTen - An integer.
 * @param {(function(number):number)=} mode - The rounding mode to use (default is `Math.round`).
 * @return {number} The rounded number.
 */
function roundToDecimal(number, powerOfTen, mode) {
    var FRACTION = 0;
    var EXPONENT = 1;
    /**
     * A decimal arithmetic shift.
     * @param {number} value - The number to shift.
     * @param {number} offset - The exponent offset.
     * @return {number} The shifted number.
     */
    function shift(value, offset) {
        /* This technique is credited to Lam Wei Li. */
        var parts = String(value).split("e");
        var exponent = Number(parts[EXPONENT] || "0") + offset;
        return Number(parts[FRACTION] + "e" + exponent);
    }
    mode = mode || Math.round;
    return shift(mode(shift(number, -powerOfTen)), powerOfTen);
}

roundToDecimal(3456.3456, -1, Math.ceil); // 3456.4
roundToDecimal(3456.3456, -2, Math.floor); // 3456.34
roundToDecimal(3456.3456,  1, Math.round); // 3460
roundToDecimal(3456.3456,  2, Math.trunc); // 3400