js-rgb-hex-color.js

function leftZeroPad(value, maxlength) {
  while (value.length < maxlength) {
    value = '0' + value;
  }
  return value;
}

function boundedValue(minvalue, maxvalue) {
  return function(value) {
    return Math.max(minvalue, Math.min(maxvalue, +value || 0));
  }
}

function colorValue(value) {
  var hex = boundedValue(0, 255)(value).toString(16);
  return leftZeroPad(hex, 2);
}

function rgbToHex(red, green, blue) {
  red = colorValue(red);
  green = colorValue(green);
  blue = colorValue(blue);
  
  return '#' + red + green + blue;
}

console.log(rgbToHex()); // "#000000"
console.log(rgbToHex(255)); // "#ff0000"
console.log(rgbToHex(255, 128)); // "#ff8000"
console.log(rgbToHex(255, 128, 64)); // "#ff8040"