The code you've written is as follows

const rgb2hex = ([r, g, b]) =>
  `#${(1 << 24) + (r << 16) + (g << 8) + b}`.toString(16).substr(1);

console.log(rgb2hex([76, 11, 181]));
// #4c0bb5

Which the desired output is #4c0bb5 but what we get is 21760949 .

the correct way of writing the code is as follows

function rgbToHex(r, g, b) {
  return "#" + ((1 << 24) + (r << 16) + (g << 8) + b).toString(16).slice(1);
}

console.log(rgbToHex(0, 51, 255)); // #0033ff

And the reason it works because we are converting the number part to hex.

And also thanks for your awesome code you've share. i learned a lot from them.
Happy Coding! 😃