TypeScript Advanced Patterns

Discriminated Unions

Model state machines and mutually exclusive variants with a shared literal discriminant:

type Result<T, E = Error> = { success: true; data: T } | { success: false; error: E };

function handle(result: Result<User>) {
    if (result.success) {
        console.log(result.data.name); // narrowed to { data: User }
    } else {
        console.error(result.error); // narrowed to { error: Error }
    }
}

Prefer discriminated unions over optional fields when states are mutually exclusive.