Exercise: Model Structured API States with Discriminated Unions

We’ve seen how discriminated unions give us precision. Now let’s take that precision and put it to work.

The problem

We’re building a frontend that calls an API to fetch user profile data. The server doesn’t just return raw values—it sends structured responses that reflect the state of the request. This is a perfect use case for discriminated unions.

In this exercise, you’ll model those responses using a type-safe discriminated union.

The API can return one of three structured outcomes:

1. A success response that includes:

   1. A status field with the literal value "success".

   2. A user object with:

      1. id (number).

      2. name (string).

      3. email (string).

2. A failure response that includes:

   1. A status field with the literal value "failure".

   2. A message string explaining what went wrong.

3. A pending response that includes:

   1. A status field with the literal value "pending".

   2. A retryAfter timestamp in milliseconds (number).

Your tasks:

• Define a discriminated union type called ProfileApiResponse that captures all three cases above.

• Write a function handleApiResponse(response: ProfileApiResponse): string that returns a human-readable message depending on the response type.

• Use a switch on the discriminant (status) to narrow the type.

• Enforce exhaustiveness with a never check in the switch statement.

• Test your implementation with example calls for all three response types.

Good luck!