index.js

// TODO: Remove this ignore comment.
// eslint-disable-next-line no-mixed-operators
const toHex = (red, green, blue, alpha) => ((blue | green << 8 | red << 16) | 1 << 24).toString(16).slice(1) + alpha;

export default function rgbHex(red, green, blue, alpha) {
	let isPercent = (red + (alpha || '')).toString().includes('%');

	if (typeof red === 'string' && !green) { // Single string parameter.
		const parsed = parseCssRgbString(red);
		if (!parsed) {
			throw new TypeError('Invalid or unsupported color format.');
		}

		isPercent = false;
		[red, green, blue, alpha] = parsed;
	} else if (alpha !== undefined) {
		alpha = Number.parseFloat(alpha);
	}

	if (typeof red !== 'number' ||
		typeof green !== 'number' ||
		typeof blue !== 'number' ||
		red > 255 ||
		green > 255 ||
		blue > 255
	) {
		throw new TypeError('Expected three numbers below 256');
	}

	if (typeof alpha === 'number') {
		if (!isPercent && alpha >= 0 && alpha <= 1) {
			alpha = Math.round(255 * alpha);
		} else if (isPercent && alpha >= 0 && alpha <= 100) {
			alpha = Math.round(255 * alpha / 100);
		} else {
			throw new TypeError(`Expected alpha value (${alpha}) as a fraction or percentage`);
		}

		alpha = (alpha | 1 << 8).toString(16).slice(1); // eslint-disable-line no-mixed-operators
	} else {
		alpha = '';
	}

	return toHex(red, green, blue, alpha);
}

const parseCssRgbString = input => {
	const parts = input.replace(/rgba?\(([^)]+)\)/, '$1').split(/[,\s/]+/).filter(Boolean);
	if (parts.length < 3) {
		return;
	}

	const parseValue = (value, max) => {
		value = value.trim();

		if (value.endsWith('%')) {
			return Math.min(Number.parseFloat(value) * max / 100, max);
		}

		return Math.min(Number.parseFloat(value), max);
	};

	const red = parseValue(parts[0], 255);
	const green = parseValue(parts[1], 255);
	const blue = parseValue(parts[2], 255);
	let alpha;

	if (parts.length === 4) {
		alpha = parseValue(parts[3], 1);
	}

	return [red, green, blue, alpha];
};