Form Patterns

Stack Decision

React Hook Form + Zod

Use zodResolver(schema) with useForm. Register inputs with {...register('field')}. Display errors.field.message with role='alert' and aria-describedby.

Multi-Step Wizard

Track current step in state. Merge step data on each submit. Each step is its own RHF instance with defaultValues from accumulated data.

Server Actions (Next.js)

Mark action files 'use server'. Parse FormData with Zod's safeParse. Return { errors } on failure. Client uses useActionState(action, initialState) returning [state, formAction, isPending].

File Upload

Use useRef for hidden file input. Handle drag-and-drop via onDrop/onDragOver. Transfer dropped files to input via new DataTransfer() + dt.items.add(file) for form submission.

Validation UX

Use mode: 'onBlur' for inline validation. Debounce async validators (username availability). Prefer inline-on-blur for most fields, on-submit for short forms.

Accessibility Checklist

• Every <input> has a <label> with matching htmlFor/id
• Error messages use role="alert" and link via aria-describedby
• Required fields use aria-required="true" (not just required attribute)
• Focus the first error field on submit failure
• Disabled submit buttons still explain why (tooltip or adjacent text)

Gotchas

• Controlled vs uncontrolled: React Hook Form is uncontrolled by default; Controller wraps controlled components (Select, DatePicker). Don't mix register with value prop
• FormData with checkboxes: unchecked checkboxes are absent from FormData, not false -- parse with formData.get('field') === 'on'
• File input reset: setting input.value = '' is the only way to clear; React state won't do it
• Server Action errors: useActionState replaces useFormState in React 19; always return structured error objects, not thrown errors
• Zod .transform(): transforms run after validation; z.coerce.number() for FormData string-to-number

Cross-References

• frontend:react-state-management -- managing form state alongside global state
• frontend:nextjs-app-router-patterns -- server actions, revalidation, and progressive enhancement
• languages:pydantic-and-data-validation -- server-side schema validation mirroring Zod patterns