function rgba2hex(rgba) {
  rgba = rgba.match(
    /^rgba?[\s+]?\([\s+]?(\d+)[\s+]?,[\s+]?(\d+)[\s+]?,[\s+]?(\d+)[\s+]?/i
  );
  return rgba && rgba.length === 4
    ? "#" +
        ("0" + parseInt(rgba[1], 10).toString(16)).slice(-2) +
        ("0" + parseInt(rgba[2], 10).toString(16)).slice(-2) +
        ("0" + parseInt(rgba[3], 10).toString(16)).slice(-2)
    : "";
}
// examples
console.log(rgba2hex('rgba(240, 240, 240, 0.5)'));
console.log(rgba2hex('rgba(40, 20, 80, 1.0)'));