Chapter 28: Utility & State Types

Core Idea

A handful of type aliases underpin the whole type-inference story: DeepKeys/DeepValue compute dot-path field names and their value types from defaultValues, BaseFormState/DerivedFormState describe what's actually in form.state, and Updater/ValidationError/ValidationMeta are small supporting shapes used throughout the API.

Reference Tables

Inference primitives

Type	Purpose
DeepKeys<T>	Computes every valid dot-path field name for a (possibly deeply nested) object type T — this is what makes a typo in form.Field's name prop a compile error
DeepValue<T, Path>	Given a T and a DeepKeys<T> path, resolves the value type at that path — what makes field.state.value correctly typed per-field

Form state

Field (on BaseFormState, extended by DerivedFormState)	Purpose
values	Current values of every field
errorMap	Errors keyed by validation trigger (onChange/onBlur/onSubmit/onDynamic, ch027)
fieldMetaBase	Per-field metadata, excluding derived/computed properties
formGroupStateBase	Submission lifecycle state for any mounted FormGroup (ch012)
isSubmitting	True while a submission is in flight
isSubmitted	Resets to false at the start of each new submission attempt
isSubmitSuccessful	Whether the most recent submission completed without error
isValidating	True while the form or any field is mid-validation
submissionAttempts	Running count of submit attempts (useful input to revalidateLogic's "after first submit" logic, ch009)
validationMetaMap	Internal validation-tracking metadata
(DerivedFormState adds) isValid / canSubmit	Computed convenience booleans layered on top of the base state fields above

Small supporting types

Type	Purpose
Updater<T>	The general "either a new T, or a function from old T to new T" shape accepted by every set* method (setFieldValue, setValue, etc.)
UpdaterFn<T>	The function-form half of Updater<T> specifically
ValidationError	Typed as unknown on purpose — any truthy value is a valid error (ch016 Custom Errors)
ValidationMeta	Metadata carried alongside a validation run (e.g. for cancellation of superseded async validations)
AnyFieldMeta	The type-erased field-metadata shape (isTouched/isDirty/errors/etc.) used where a field's exact generic parameters aren't known

Key Takeaways

1. DeepKeys/DeepValue are why form.Field name="typo" is a compile-time error instead of a silent runtime no-op — understanding these two types explains most of the library's "magical" type safety.
2. isValid/canSubmit are derived (DerivedFormState), not stored directly — they're computed from the base error/validation state on every read, not fields you'd manually set.
3. Every set* method across FormApi/FieldApi (ch026) accepts an Updater<T> — you can always pass either a plain new value or an updater function, exactly like useState's setter overload.

Connects To

• Ch005 TypeScript: the strict/version requirements these inference types depend on.
• Ch026 Field & Form API Classes: the classes whose state this chapter's types describe.
• Ch016 Custom Errors: why ValidationError is deliberately typed as unknown.