function map (num, in_min, in_max, out_min, out_max) {
  return (num - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

var num = 5;
console.log(map(num, 0, 255, 0, 1)); // 0.0196078431372549
var num = 150
console.log(map(num, 0, 255, 0, 1)); // 0.5882352941176471