TypeScriptSau khi handle tất cả cases trong discriminated union, assign default case sang variable type never.
Nếu có case chưa handle, TS báo lỗi.
typescript
type Action =
| { type: 'INCREMENT' }
| { type: 'DECREMENT' }
| { type: 'RESET' };
function reducer(state: number, action: Action): number {
switch (action.type) {
case 'INCREMENT': return state + 1;
case 'DECREMENT': return state - 1;
case 'RESET': return 0;
default:
// Nếu thêm 'SET' vào Action mà không handle, dòng này báo lỗi
const exhaustive: never = action;
throw new Error(`Unhandled action: ${exhaustive}`);
}
}
// Hoặc dùng helper function
function assertNever(x: never): never {
throw new Error('Unexpected value: ' + x);
}Pattern này đảm bảo thêm case mới vào union sẽ bị catch ngay tại compile time.
After handling all cases of a discriminated union, assign the default case to a variable of type never.
If any case is unhandled, TS reports an error.
typescript
type Action =
| { type: 'INCREMENT' }
| { type: 'DECREMENT' }
| { type: 'RESET' };
function reducer(state: number, action: Action): number {
switch (action.type) {
case 'INCREMENT': return state + 1;
case 'DECREMENT': return state - 1;
case 'RESET': return 0;
default:
// If 'SET' is added to Action without handling it, this line errors
const exhaustive: never = action;
throw new Error(`Unhandled action: ${exhaustive}`);
}
}
// Or use a helper function
function assertNever(x: never): never {
throw new Error('Unexpected value: ' + x);
}This pattern ensures that adding a new case to the union is caught immediately at compile time.