57 lines
2.5 KiB
Markdown
57 lines
2.5 KiB
Markdown
---
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title: Keep shouldUnregister Off Unless Hidden Fields Must Leave the Payload
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impact: HIGH
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impactDescription: prevents silently dropping values the user already entered
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tags: formcfg, should-unregister, dynamic-forms, conditional-fields
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---
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## Keep shouldUnregister Off Unless Hidden Fields Must Leave the Payload
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The default (`shouldUnregister: false`) keeps a field's value in form state after its input unmounts. That is the right default and it is not a memory problem — the retained data is a few strings per field. `shouldUnregister: true` is a **submission-shape** option: it removes unmounted fields from the values object entirely. Reaching for it to "clean up" a multi-step wizard silently deletes everything the user entered on step 1 the moment they advance to step 2.
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**Incorrect (wizard loses step 1 the moment step 2 renders):**
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```typescript
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function OnboardingWizard() {
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const [step, setStep] = useState(1)
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const { register, handleSubmit } = useForm<OnboardingData>({
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shouldUnregister: true, // personalName is dropped as soon as step 1 unmounts
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defaultValues: { personalName: '', companyName: '' },
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})
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return (
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<form onSubmit={handleSubmit(completeOnboarding)}>
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{step === 1 && <input {...register('personalName')} />}
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{step === 2 && <input {...register('companyName')} />}
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<button type="button" onClick={() => setStep(2)}>Next</button>
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</form>
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)
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}
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```
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**Correct (default retention — every step survives to submit):**
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```typescript
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function OnboardingWizard() {
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const [step, setStep] = useState(1)
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const { register, handleSubmit } = useForm<OnboardingData>({
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defaultValues: { personalName: '', companyName: '' },
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})
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return (
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<form onSubmit={handleSubmit(completeOnboarding)}>
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{step === 1 && <input {...register('personalName')} />}
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{step === 2 && <input {...register('companyName')} />}
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<button type="button" onClick={() => setStep(2)}>Next</button>
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</form>
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)
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}
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```
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**When `shouldUnregister: true` is the right call:**
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- A discriminated payload where the hidden branch's keys must be absent, not empty — e.g. a "Business" account sends `taxId` and a "Personal" one must not send the key at all
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- A backend that treats a present-but-empty key differently from an absent one
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- Set it per field via `register('taxId', { shouldUnregister: true })` rather than form-wide, so the rest of the form keeps the safe default
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Reference: [useForm - shouldUnregister](https://react-hook-form.com/docs/useform)
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