function rgb2hex(rgb) {

 rgb = rgb.match(/^rgb\((\d+),(\d+),(\d+)\)$/);

 function hex(x) {

  return ("0" + parseInt(x).toString(16)).slice(-2);

 }

 return "#" + hex(rgb[1]) + hex(rgb[2]) + hex(rgb[3]);

}

var rgb = 'rgb(255,255,255)';

console.log(rgb2hex(rgb));