Engineering practice · 30 MIN

Model legal state transitions

A reducer makes transition rules reviewable and testable.

Represent a request as idle, loading, success or error. An exhaustive switch forces you to handle new event variants when using the TypeScript checker. Runtime logic must also reject illegal transitions: a success response should not overwrite an idle state. In a real application include a request ID so late responses cannot replace newer work.

Treat the function as a small service: define a contract, maintain an invariant, and test the boundaries.

Read the example

type Status = "idle" | "loading";
const status: Status = "loading";
console.log(status);

loading

Your challenge

Input is an array containing "start", "success", "fail", "reset". Begin idle. start changes idle/error/success to loading; success/fail only apply while loading; reset always returns idle. Return the final state.

Solution cost: O(n) events. time · O(1) space

Common trap

Compile-time exhaustiveness does not prevent a response from an older request winning a race.

Further reading: TypeScript: discriminated unions