Randomly picking an object property based off of its value (lottery)?

I have a object like so:

{
  "user1": 200,
  "user2": 50,
  "user3": 250,
  "user4": 100
}

I would want to randomly pick one of these users based off of the value associated to their property. So in this scenario, user1 should have a 33.33% chance of winning, user2 should have 8.33% and so on. How could I do that?