fetch_with_union_types.ts

export function main() {
  let userRequest: Data = { type: "Loading" };

  fetchData()
    .then(response => {
      userRequest = { type: "Success", data: response };
    })
    .catch(error => {
      userRequest = { type: "Failure", error };
    });
}

function render(userRequest: Data) {
  switch (userRequest.type) {
    case "Loading":
      return "Loading...";

    case "Success":
      return `Hello, ${userRequest.data.username}`;

    case "Failure":
      return "Could not fetch data!";
  }
}