next.js/packages/next/src/client/form.tsx

'use client'

import { type FormEvent, useContext, forwardRef } from 'react'
import { addBasePath } from './add-base-path'
import { RouterContext } from '../shared/lib/router-context.shared-runtime'
import type { NextRouter } from './router'
import {
  checkFormActionUrl,
  createFormSubmitDestinationUrl,
  DISALLOWED_FORM_PROPS,
  hasReactClientActionAttributes,
  hasUnsupportedSubmitterAttributes,
  type FormProps,
} from './form-shared'

export type { FormProps }

const Form = forwardRef<HTMLFormElement, FormProps>(function FormComponent(
  { replace, scroll, prefetch: prefetchProp, ...props },
  ref
) {
  const router = useContext(RouterContext)

  const actionProp = props.action
  const isNavigatingForm = typeof actionProp === 'string'

  // Validate `action`
  if (process.env.NODE_ENV === 'development') {
    if (isNavigatingForm) {
      checkFormActionUrl(actionProp, 'action')
    }
  }

  // Validate `prefetch`
  if (process.env.NODE_ENV === 'development') {
    if (prefetchProp !== undefined) {
      console.error(
        'Passing `prefetch` to a <Form> has no effect in the pages directory.'
      )
    }
  }

  // Validate `scroll` and `replace`
  if (process.env.NODE_ENV === 'development') {
    if (!isNavigatingForm && (replace !== undefined || scroll !== undefined)) {
      console.error(
        'Passing `replace` or `scroll` to a <Form> whose `action` is a function has no effect.\n' +
          'See the relevant docs to learn how to control this behavior for navigations triggered from actions:\n' +
          '  `router.replace()` - https://nextjs.org/docs/pages/api-reference/functions/use-router#routerreplace\n'
      )
    }
  }

  // Clean up any unsupported form props (and warn if present)
  for (const key of DISALLOWED_FORM_PROPS) {
    if (key in props) {
      if (process.env.NODE_ENV === 'development') {
        console.error(`<Form> does not support changing \`${key}\`.`)
      }
      delete (props as Record<string, unknown>)[key]
    }
  }

  if (!isNavigatingForm) {
    return <form {...props} ref={ref} />
  }

  const actionHref = addBasePath(actionProp)

  return (
    <form
      {...props}
      ref={ref}
      action={actionHref}
      onSubmit={(event) =>
        onFormSubmit(event, {
          router,
          actionHref,
          replace,
          scroll,
          onSubmit: props.onSubmit,
        })
      }
    />
  )
})

export default Form

function onFormSubmit(
  event: FormEvent<HTMLFormElement>,
  {
    actionHref,
    onSubmit,
    replace,
    scroll,
    router,
  }: {
    actionHref: string
    onSubmit: FormProps['onSubmit']
    replace: FormProps['replace']
    scroll: FormProps['scroll']
    router: NextRouter | null
  }
) {
  if (typeof onSubmit === 'function') {
    onSubmit(event)

    // if the user called event.preventDefault(), do nothing.
    // (this matches what Link does for `onClick`)
    if (event.defaultPrevented) {
      return
    }
  }

  if (!router) {
    // Form was somehow used outside of the router (but not in app/, the implementation is forked!).
    // We can't perform a soft navigation, so let the native submit handling do its thing.
    return
  }

  const formElement = event.currentTarget
  const submitter = (event.nativeEvent as SubmitEvent).submitter

  let action = actionHref

  if (submitter) {
    // this is page-router-only, so we don't need to worry about false positives
    // from the attributes that react adds for server actions.
    if (hasUnsupportedSubmitterAttributes(submitter)) {
      return
    }

    // client actions have `formAction="javascript:..."`. We obviously can't prefetch/navigate to that.
    if (hasReactClientActionAttributes(submitter)) {
      return
    }

    // If the submitter specified an alternate formAction,
    // use that URL instead -- this is what a native form would do.
    // NOTE: `submitter.formAction` is unreliable, because it will give us `location.href` if it *wasn't* set
    // NOTE: this should not have `basePath` added, because we can't add it before hydration
    const submitterFormAction = submitter.getAttribute('formAction')
    if (submitterFormAction !== null) {
      if (process.env.NODE_ENV === 'development') {
        checkFormActionUrl(submitterFormAction, 'formAction')
      }
      action = submitterFormAction
    }
  }

  const targetUrl = createFormSubmitDestinationUrl(action, formElement)

  // Finally, no more reasons for bailing out.
  event.preventDefault()

  const method = replace ? 'replace' : 'push'
  const targetHref = targetUrl.href // TODO: will pages router be happy about an absolute URL here?

  // TODO(form): Make this use a transition so that pending states work
  //
  // Unlike the app router, pages router doesn't use startTransition,
  // and can't easily be wrapped in one because of implementation details
  // (e.g. it doesn't use any react state)
  // But it's important to have this wrapped in a transition because
  // pending states from e.g. `useFormStatus` rely on that.
  // So this needs some follow up work.
  router[method](targetHref, undefined, { scroll })
}