Javascript color scale from 0% to 100%, rendering it from red to yellow to green

color-scale.js

// License: MIT - https://opensource.org/licenses/MIT
// Author: Michele Locati <michele@locati.it>
// Source: https://gist.github.com/mlocati/7210513
function perc2color(perc) {
	var r, g, b = 0;
	if(perc < 50) {
		r = 255;
		g = Math.round(5.1 * perc);
	}
	else {
		g = 255;
		r = Math.round(510 - 5.10 * perc);
	}
	var h = r * 0x10000 + g * 0x100 + b * 0x1;
	return '#' + ('000000' + h.toString(16)).slice(-6);
}

Tracked a bug down in my own code related to this gist, if perc is not a number, you get NaN in the output. Adding this as a defense and modernising;

function perc2color(perc) {
    let cleanPerc = parseFloat(perc);
    if (isNaN(cleanPerc)) cleanPerc = 0;

    cleanPerc = Math.max(0, Math.min(100, cleanPerc));

    let r = 0, g = 0, b = 0;
    if (cleanPerc < 50) {
        r = 255;
        g = Math.round(5.1 * cleanPerc);
    } else {
        g = 255;
        r = Math.round(510 - 5.10 * cleanPerc);
    }
    
    const h = (r << 16) + (g << 8) + b;
    return '#' + h.toString(16).padStart(6, '0');
}