Parse hex colors with template literal types

// Hex color strings are constrained by a template literal type and parsed.
type HexColor = `#${string}`;
type Rgb = readonly [red: number, green: number, blue: number];

function parseHex(color: HexColor): Rgb {
  let hex = color.slice(1);
  if (hex.length === 3) hex = [...hex].map((char) => char + char).join('');
  if (hex.length !== 6 || !/^[0-9a-f]+$/i.test(hex)) {
    throw new TypeError(`invalid hex color: ${color}`);
  }
  return [
    parseInt(hex.slice(0, 2), 16),
    parseInt(hex.slice(2, 4), 16),
    parseInt(hex.slice(4, 6), 16),
  ];
}

function withAlpha(color: HexColor, alpha = 1): string {
  const [red, green, blue] = parseHex(color);
  return `rgba(${red}, ${green}, ${blue}, ${alpha})`;
}

const brand: HexColor = '#4f8';
console.log(withAlpha(brand)); // rgba(68, 255, 136, 1)