$app/forms

import { applyAction(alias) function applyAction<Success extends Record<string, unknown> | undefined, Failure extends Record<string, unknown> | undefined>(result: ActionResult<Success, Failure>): Promise<void>
import applyAction
This action updates the form property of the current page with the given data and updates page.status.
In case of an error, it redirects to the nearest error page.
reference, deserialize(alias) function deserialize<Success extends Record<string, unknown> | undefined, Failure extends Record<string, unknown> | undefined>(result: string): ActionResult<Success, Failure>
import deserialize
Use this function to deserialize the response from a form submission.
Usage:
import { deserialize } from '$app/forms';

async function handleSubmit(event) {
  const response = await fetch('/form?/action', {
    method: 'POST',
    body: new FormData(event.target)
  });

  const result = deserialize(await response.text());
  // ...
}reference, enhance(alias) function enhance<Success extends Record<string, unknown> | undefined, Failure extends Record<string, unknown> | undefined>(form_element: HTMLFormElement, submit?: SubmitFunction<Success, Failure>): {
    destroy(): void;
}
import enhance
This action enhances a <form> element that otherwise would work without JavaScript.
The submit function is called upon submission with the given FormData and the action that should be triggered.
If cancel is called, the form will not be submitted.
You can use the abort controller to cancel the submission in case another one starts.
If a function is returned, that function is called with the response from the server.
If nothing is returned, the fallback will be used.
If this function or its return value isn't set, it

falls back to updating the form prop with the returned data if the action is on the same page as the form
updates page.status
resets the <form> element and invalidates all data in case of successful submission with no redirect response
redirects in case of a redirect response
redirects to the nearest error page in case of an unexpected error

If you provide a custom function with a callback and want to use the default behavior, invoke update in your callback.
It accepts an options object

reset: false if you don't want the <form> values to be reset after a successful submission
invalidateAll: false if you don't want the action to call invalidateAll after submission

form_element 
The form element
submit 
Submit callback
reference } from '$app/forms';

import { deserialize(alias) function deserialize<Success extends Record<string, unknown> | undefined, Failure extends Record<string, unknown> | undefined>(result: string): ActionResult<Success, Failure>
import deserialize
Use this function to deserialize the response from a form submission.
Usage:
import { deserialize } from '$app/forms';

async function handleSubmit(event) {
  const response = await fetch('/form?/action', {
    method: 'POST',
    body: new FormData(event.target)
  });

  const result = deserialize(await response.text());
  // ...
}reference } from '$app/forms';

async function handleSubmitfunction handleSubmit(event: any): Promise<void>(event(parameter) event: any) {
	const responseconst response: Response = await fetchfunction fetch(input: string | URL | Request, init?: RequestInit): Promise<Response> (+1 overload)
MDN Reference
('/form?/action', {
		method(property) RequestInit.method?: string | undefined
A string to set request's method.
: 'POST',
		body(property) RequestInit.body?: BodyInit | null | undefined
A BodyInit object or null to set request's body.
: new FormDatavar FormData: new (form?: HTMLFormElement, submitter?: HTMLElement | null) => FormData
The FormData interface provides a way to construct a set of key/value pairs representing form fields and their values, which can be sent using the fetch(), XMLHttpRequest.send() or navigator.sendBeacon() methods. It uses the same format a form would use if the encoding type were set to "multipart/form-data".
MDN Reference
(event(parameter) event: any.targetany)
	});

	const resultconst result: ActionResult<Record<string, unknown> | undefined, Record<string, unknown> | undefined> = deserialize(alias) deserialize<Record<string, unknown> | undefined, Record<string, unknown> | undefined>(result: string): ActionResult<Record<string, unknown> | undefined, Record<string, unknown> | undefined>
import deserialize
Use this function to deserialize the response from a form submission.
Usage:
import { deserialize } from '$app/forms';

async function handleSubmit(event) {
  const response = await fetch('/form?/action', {
    method: 'POST',
    body: new FormData(event.target)
  });

  const result = deserialize(await response.text());
  // ...
}reference(await responseconst response: Response.text(method) Body.text(): Promise<string>
MDN Reference
());
	// ...
}