Intermediate · 14 min

Exhaustive state checks

Use never to make an omitted union case a compile-time error.

Handle every alternative

After every possible union member has been handled, the remaining value has type never. Assigning it to a never variable makes a newly added, unhandled case visible to the compiler. Do not cast a value to never to silence this check.

type Mode = "read" | "write";
function label(mode: Mode): string {
  switch (mode) {
    case "read": return "Reading";
    case "write": return "Writing";
    default: { const missing: never = mode; return missing; }
  }
}
console.log(label("write"));

Output

Writing

Keep a runtime boundary

An exhaustive switch checks values already described by a union. It does not validate external data. A throwing helper is useful for unexpected runtime values, but input still needs validation before you treat it as a trusted union.

function unreachable(value: never): never {
  throw new Error("Unexpected state: " + String(value));
}
type State = { kind: "idle" } | { kind: "done"; count: number };
function count(state: State): number {
  switch (state.kind) {
    case "idle": return 0;
    case "done": return state.count;
    default: return unreachable(state);
  }
}
console.log(count({ kind: "done", count: 3 }));

Output

Put it into practice

Read the types and predict the output before running the examples.
Fix the task without using "any" or a type assertion, then check both practice questions.

Try it yourself

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Handle the added state

Add the missing "paused" case and return "Paused". Keep the never check so future omissions remain visible.

Your TypeScript code

Show solution

type State = "idle" | "active" | "paused";
function label(state: State): string {
  switch (state) {
    case "idle": return "Ready";
    case "active": return "Working";
    case "paused": return "Paused";
    default: { const missing: never = state; return missing; }
  }
}
const result = label("paused");
console.log(result);

Adding the missing branch leaves no possible State in the default branch. The never assignment now compiles and continues to detect future omissions.

Practice