Document useForm -> onSubmit hook

[ivan-kleshnin]

Arguments of onSubmit fn argument of useForm are not documented. For people who don't use React actions (use client-only forms) onSubmit becomes a crucial piece.

From my quick experiments:

useForm({
  ...
  onSubmit(event: ..., context: Context) {
    ...
  }
})

type Context = {
  action: string
  encType: string // enum
  formData: FormData
  method: string // enum
  submission?: { //
    status: 'success' 
    payload: {…}, // -- I assume it's the string-only original form data
    value: {…} // -- looks like parsed zod result of `Schema` type. Requires status == "success" condition 🤔
    reply: ƒ // -- not sure what it is
  } | {status: "error", ...}
}

^ A pretty complex object, we shouldn't guess it's structure. Here's what I ended-up with:

onSubmit(event, {submission}) {
  event.preventDefault()
  if (submission?.status == "success") { // unfortunately, here we have to add a check which will never fail if `onValidate` is provided
    console.log("value:", submission.value)
  }
}