index.js

// Returns a random integer between min (included) and max (included)
// Math.floor because Math.round() yields non-uniform distribution
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random
const getRandomIntInclusive = (min, max) => {
	return Math.floor(Math.random() * (max - min + 1)) + min;
};

export default () => {
	// Seed the primary color
	const hue = getRandomIntInclusive(1, 359);
	// Comment out above line and uncomment below to test specific values
	// const hue = 9; // 128 173 (test values)

	// Derive complement (secondary) hue
	var complement = hue > 180 ? hue - 180 : hue + 179;

	// analogous (tertiary) hue
	var analogous;
	switch (hue) {
		case hue < 135:
			analogous = hue + 224;
			break;
		case hue > 224:
			analogous = hue - 223;
			break;
		default:
			var shift = hue + 225;
			analogous = shift > 359 ? shift - 359 : shift;
	}

	// Seed random saturation and lightness values within acceptable parameters
	const saturation = `${getRandomIntInclusive(80, 100)}%`;
	// highlight
	const xlight = `${getRandomIntInclusive(84, 92)}%`;
	// more extreme lightness
	const lighter = `${getRandomIntInclusive(76, 84)}%`;
	// very light or white text
	const lightness = `${getRandomIntInclusive(64, 72)}%`;
	// mid value used for lightness and saturation
	const midrange = `${getRandomIntInclusive(48, 64)}%`;
	// dark highlight
	const lowmid = `${getRandomIntInclusive(28, 36)}%`;
	// very dark or black text
	const darkness = `${getRandomIntInclusive(16, 24)}%`;
	// more extreme darkness
	const darker = `${getRandomIntInclusive(0, 12)}%`;

	// mixing values
	return {
		hue: hue,
		complement: complement,
		analogous: analogous,
		saturation: saturation,
		xlight: xlight,
		lighter: lighter,
		lightness: lightness,
		midrange: midrange,
		lowmid: lowmid,
		darkness: darkness,
		darker: darker,
	};
};