Quick pattern I keep using for API responses that can be either successful or error states:
type ApiResponse<T> =
| { success: true; data: T }
| { success: false; error: string };
// Helper to narrow the type
function isSuccess<T>(response: ApiResponse<T>): response is { success: true; data: T } {
return response.success === true;
}
// Usage
const result: ApiResponse<User> = await fetchUser();
if (isSuccess(result)) {
// TypeScript knows result.data exists here
console.log(result.data.name);
} else {
// And knows result.error exists here
console.error(result.error);
}
The type guard makes the discriminated union much nicer to work with. No need for optional chaining or non-null assertions.
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