オブジェクトにプロパティ追加する

Object.prototype.rain = "雨です";
Object.prototype.sunny = "晴れです";
Object.prototype.cloudy = "曇りです";
today_weather = new Object();

num = prompt("何か数字入れて");

if(num == 1){
  alert(today_weather.rain);
}else if(num == 2){
  alert(today_weather.sunny);
}else if(num ==3){
  alert(today_weather.cloudy);
}else{
  new_weather = prompt("新しい天気は?");
  Object.prototype.another = new_weather;
  alert(today_weather.another);
}