hex.js

const numWithOpacity = (nums, by) =>
  "#" +
  nums.map((x) => Math.round(x + (255 - x) * (1 - by)).toString(16)).join("");

const hexWithOpacity = (rgbHex, by) =>
  numWithOpacity(
    [rgbHex.slice(1, 3), rgbHex.slice(3, 5), rgbHex.slice(5, 7)].map((hex) =>
      parseInt(hex, 16)
    ),
    by
  );

// console.log(hexWithOpacity("#000000", 0.26));
console.log(numWithOpacity([0, 0, 0], 0.54));
// console.log(numWithOpacity([33, 33, 33], 0.26));