Summary - Exhaustive check
- TypeScript course
- Advanced
- Lesson 36/57
- 4 min
Write a safe state manager with exhaustive check.
Summary - Exhaustive check
Exhaustive check guarantees that switches processing union members are all exhaustive. With assertNever, the default branch takes a never parameter; if a case is missing, the compiler errors. This forces all processing sites to be updated when a new state is added in state managers.
type State = "idle" | "loading" | "ready" | "error";
function assertNever(x: never): never {
throw new Error("Unexpected: " + x);
}
function transition(s: State): string {
switch (s) {
case "idle": return "Start";
case "loading": return "Loading";
case "ready": return "Ready";
case "error": return "Error";
default: return assertNever(s);
}
}Practice Write a state manager
Define an assertNever(x: never): never function (with throw new Error("Unexpected") inside). Define the State = "idle" | "loading" | "ready" | "error" union type. Write a transition(s: State): string function using switch (s): for idle return "Başlat", for loading return "Yükleniyor", for ready return "Hazır", for error return "Hata", for default write return assertNever(s);. Print results via console.log(transition("ready")) and console.log(transition("error")).
Starter code
// assertNever fonksiyonunu tanımla (never parametre, never dönüş, throw içersin)
// State union türünü tanımla (idle, loading, ready, error)
// transition fonksiyonunu switch ile yaz, default'ta assertNever çağır
// "ready" ve "error" durumlarının sonuçlarını yazdırSample solution
function assertNever(x: never): never {
throw new Error("Unexpected");
}
type State = "idle" | "loading" | "ready" | "error";
function transition(s: State): string {
switch (s) {
case "idle":
return "Başlat";
case "loading":
return "Yükleniyor";
case "ready":
return "Hazır";
case "error":
return "Hata";
default:
return assertNever(s);
}
}
console.log(transition("ready"));
console.log(transition("error"));Expected output
Hazır Hata
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What happens when a new state is added to a state manager?
- Existing cases are deleted
- Missing cases in all switches cause compiler errors Correct answer
- assertNever is called automatically
- A runtime error is thrown
Explanation: When a new state is added, in switches with missing cases the variable in the default branch is not never; the assertNever call gives a compile error and reports the places to update.
2 Handle the error state
function assertNever(x: never): never {
throw new Error("Unexpected");
}
type State = "idle" | "loading" | "ready" | "error";
function transition(s: State): string {
switch (s) {
case "idle": return "Başlat";
case "loading": return "Yükleniyor";
case "ready": return "Hazır";
case ____: return "Hata";
default: return assertNever(s);
}
}function assertNever(x: never): never {
throw new Error("Unexpected");
}
type State = "idle" | "loading" | "ready" | "error";
function transition(s: State): string {
switch (s) {
case "idle": return "Başlat";
case "loading": return "Yükleniyor";
case "ready": return "Hazır";
case "error": return "Hata";
default: return assertNever(s);
}
}Explanation: The case "error" narrows the State union to the error member; handling this state is required for the exhaustive check.
3 Exhaustive check summary matching
| Term | Meaning |
|---|---|
assertNever(x: never) |
parameter must be never |
default branch |
remaining after all cases |
Yeni durum eklenince |
missing case errors at compile |
Derleme zamanı güvenliği |
prevents runtime errors |