Convert rgb(a) colors to hexadecimal

colortohex.js

/**
* colorToHex: Convert rgb(a) or 3-6 letters hexadecimal color code to 6 letters code, with alpha-support.
* Uses an optional background parameter to control how colors with alpha channels are converted
*
* Not supported: color names such as "red", "green" (if you want to preserve those, then modify the function)
*
* Usage examples:
* colorToHex("#3F3") -> "#33ff33"
* colorToHex("rgba(255, 0, 0, 0.5)") -> "#ff8080"
* colorToHex("rgba(255,0,0,.5)", "rgb(0,255,0)") —> "#808000"
*/
function colorToHex(color, background) {
	background = !background ? [255, 255, 255] : colorToHex(background)
		.match(/(\w){2}/g)
		.map(part => parseInt(part, 16));
	const sanitized = (color || '').toLowerCase().replace(/[\s#]/g, '');
	const format = parts => '#' + parts.map(part => ('0' + part.toString(16)).slice(-2)).join('');
	if (/^([\da-f]{3}){1,2}$/.test(sanitized)) {
		if (sanitized.length === 3) { // format 3 chars to 6
			return '#' + sanitized.split('').map(char => char + char).join('');
		}
		return '#' + sanitized;
	}
	const matches = sanitized.match(/^rgba?\(([\d\.,]*)\)/);

	if (matches) {
		const parts = matches.pop().split(',');
		const alpha = parts.length === 4 ? parseFloat(parts.pop()) : 1;
		if (alpha !== 0 && parts.length === 3) {
			return format(parts
				.map(Number)
				.map((decimal, index) => Math.round((decimal * alpha) + (background[index] * (1 - alpha))))
			);
		}
	}
	return format(background);
}