Discriminated unions
React chapter 75 · type A shows fields A · source: common pattern · unmount the other branch
The problem
A "kind" select toggles extra fields with CSS (hidden) or a ternary that leaves leftover state. Submit as company still sends personalTaxId. The schema is a flat object with everything optional.
The pattern that fixes it
Model the payload as a discriminated union: { kind: "person", nationalId } | { kind: "org", vat }. Render one branch. Unmount the other (reset its fields). Zod discriminatedUnion("kind", [...]).
Keep learning
- Previous: Native constraints vs schema
- Next: Prevent double submit
- Related: Wizard + URL step
- Related: Dependent fields
- Related: Async validation
- Related: Cross-field rules
- Related: Dirty / touched / submitCount
- Related: Focus first error
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