Hexadecimal to RGBA

hexToRgbA.ts

/**
 * @param hex HexColor #000000 #000
 * @param opacity 0-100
 * @returns `rgba(RR, GG, BB, 0.x)`
 */
export const hexToRgbA = <T extends string>(hex: HexColor<T>, opacity: number): T => {
  let c;
  if(/^#([A-Fa-f0-9]{3}){1,2}$/.test(hex)){
      c = hex.substring(1).split('');
      if(c.length == 3){
          c = [c[0], c[0], c[1], c[1], c[2], c[2]];
      }
      c = '0x' + c.join('');
      return `rgba(${[(parseInt(c) >> 16)&255, (parseInt(c) >> 8)&255, parseInt(c)&255].join(',')}, ${opacity / 100})` as T;
  }
  
  throw new Error('Bad Hex');
};

type HexDigit = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' | 'a' | 'b' | 'c' | 'd' | 'e'| 'f' | 'A' | 'B' | 'C' | 'D' | 'E'| 'F';
export type HexColor<T extends string> =
    T extends `#${HexDigit}${HexDigit}${HexDigit}${infer Rest1}`
        ? (Rest1 extends string 
            ? T // three-digit hex color
            : (
                Rest1 extends `#${HexDigit}${HexDigit}${HexDigit}`
                    ? T  // six-digit hex color
                    : string
            )
        )
        : string;