Exhaustive discriminated unions

What this means

A discriminant field gives every union member a unique literal value. An exhaustive switch can use never to expose a missing case during compilation.

In beginner terms, this topic answers one practical question: "What should I write, and why does React care about it?" Do not try to memorize the syntax first. First understand the idea, then connect the syntax to that idea.

Why it matters

State machines for requests, forms, and workflows become safer when every valid state is explicit.

When you build real React screens, this idea helps you decide where data should live, what the user should see, and what should happen after an interaction. That is why this lesson is part of the main path instead of being an optional detail.

Step by step

1. Notice the UI problem this topic solves. 2. Look at the smallest possible example. 3. Change one value and predict what should appear. 4. Run the example and compare the result with your prediction. 5. Use the practice task before moving on.

Small example

type LoadState = { status: 'idle' } | { status: 'success'; data: string } | { status: 'error'; message: string };

Common mistake

Do not copy exhaustive discriminated unions syntax without explaining what problem it solves and checking the result.

Practice task

Change one part of the example, predict the result, run it, and explain the result in your own words.

Remember this

Do not combine unrelated optional fields when a union can describe valid combinations.