feat: unshadow form and data in enhance (#9902)

* feat: Un-shadow `data` and `form` in `enhance`, warn about future deprecation in dev

* changeset

* snek

* Update .changeset/odd-crews-own.md

Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>

* Update packages/kit/test/apps/dev-only/package.json

Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>

* am not smart

* still not smart

* oops

* oof

* add deprecation notice

---------

Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>
Co-authored-by: Rich Harris <git@rich-harris.dev>

.changeset/odd-crews-own.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: unshadow `data` and `form` in `enhance` and warn about future deprecation when used in `dev` mode

packages/kit/src/runtime/app/forms.js

@@ -14,12 +14,26 @@ export function deserialize(result) {
 	return parsed;
 }
 
+/**
+ * @param {string} old_name
+ * @param {string} new_name
+ * @param {string} call_location
+ * @returns void
+ */
+function warn_on_access(old_name, new_name, call_location) {
+	if (!DEV) return;
+	// TODO 2.0: Remove this code
+	console.warn(
+		`\`${old_name}\` has been deprecated in favor of \`${new_name}\`. \`${old_name}\` will be removed in a future version. (Called from ${call_location})`
+	);
+}
+
 /** @type {import('$app/forms').enhance} */
-export function enhance(form, submit = () => {}) {
+export function enhance(form_element, submit = () => {}) {
 	if (
 		DEV &&
-		/** @type {HTMLFormElement} */ (HTMLFormElement.prototype.cloneNode.call(form)).method !==
-			'post'
+		/** @type {HTMLFormElement} */ (HTMLFormElement.prototype.cloneNode.call(form_element))
+			.method !== 'post'
 	) {
 		throw new Error('use:enhance can only be used on <form> fields with method="POST"');
 	}
@@ -35,7 +49,7 @@ export function enhance(form, submit = () => {}) {
 		if (result.type === 'success') {
 			if (reset !== false) {
 				// We call reset from the prototype to avoid DOM clobbering
-				HTMLFormElement.prototype.reset.call(form);
+				HTMLFormElement.prototype.reset.call(form_element);
 			}
 			await invalidateAll();
 		}
@@ -60,28 +74,38 @@ export function enhance(form, submit = () => {}) {
 			// We do cloneNode for avoid DOM clobbering - https://github.com/sveltejs/kit/issues/7593
 			event.submitter?.hasAttribute('formaction')
 				? /** @type {HTMLButtonElement | HTMLInputElement} */ (event.submitter).formAction
-				: /** @type {HTMLFormElement} */ (HTMLFormElement.prototype.cloneNode.call(form)).action
+				: /** @type {HTMLFormElement} */ (HTMLFormElement.prototype.cloneNode.call(form_element))
+						.action
 		);
 
-		const data = new FormData(form);
+		const form_data = new FormData(form_element);
 
 		const submitter_name = event.submitter?.getAttribute('name');
 		if (submitter_name) {
-			data.append(submitter_name, event.submitter?.getAttribute('value') ?? '');
+			form_data.append(submitter_name, event.submitter?.getAttribute('value') ?? '');
 		}
 
 		const controller = new AbortController();
 
 		let cancelled = false;
 		const cancel = () => (cancelled = true);
 
+		// TODO 2.0: Remove `data` and `form`
 		const callback =
 			(await submit({
 				action,
 				cancel,
 				controller,
-				data,
-				form,
+				get data() {
+					warn_on_access('data', 'formData', 'use:enhance submit function');
+					return form_data;
+				},
+				formData: form_data,
+				get form() {
+					warn_on_access('form', 'formElement', 'use:enhance submit function');
+					return form_element;
+				},
+				formElement: form_element,
 				submitter: event.submitter
 			})) ?? fallback_callback;
 		if (cancelled) return;
@@ -97,7 +121,7 @@ export function enhance(form, submit = () => {}) {
 					'x-sveltekit-action': 'true'
 				},
 				cache: 'no-store',
-				body: data,
+				body: form_data,
 				signal: controller.signal
 			});
 
@@ -110,21 +134,29 @@ export function enhance(form, submit = () => {}) {
 
 		callback({
 			action,
-			data,
-			form,
+			get data() {
+				warn_on_access('data', 'formData', 'callback returned from use:enhance submit function');
+				return form_data;
+			},
+			formData: form_data,
+			get form() {
+				warn_on_access('form', 'formElement', 'callback returned from use:enhance submit function');
+				return form_element;
+			},
+			formElement: form_element,
 			update: (opts) => fallback_callback({ action, result, reset: opts?.reset }),
 			// @ts-expect-error generic constraints stuff we don't care about
 			result
 		});
 	}
 
 	// @ts-expect-error
-	HTMLFormElement.prototype.addEventListener.call(form, 'submit', handle_submit);
+	HTMLFormElement.prototype.addEventListener.call(form_element, 'submit', handle_submit);
 
 	return {
 		destroy() {
 			// @ts-expect-error
-			HTMLFormElement.prototype.removeEventListener.call(form, 'submit', handle_submit);
+			HTMLFormElement.prototype.removeEventListener.call(form_element, 'submit', handle_submit);
 		}
 	};
 }

packages/kit/test/apps/basics/src/routes/actions/enhance/old-property-access/+page.server.js

@@ -0,0 +1,26 @@
+// TODO 2.0: Remove this code and corresponding tests
+export const actions = {
+	form_submit: () => {
+		return {
+			form_submit: true
+		};
+	},
+
+	form_callback: () => {
+		return {
+			form_callback: true
+		};
+	},
+
+	data_submit: () => {
+		return {
+			data_submit: true
+		};
+	},
+
+	data_callback: () => {
+		return {
+			data_callback: true
+		};
+	}
+};

packages/kit/test/apps/basics/src/routes/actions/enhance/old-property-access/+page.svelte

@@ -0,0 +1,52 @@
+<script>
+	import { enhance } from '$app/forms';
+
+	export let form;
+
+	const access_form_submit = (node) => {
+		return enhance(node, ({ form }) => {});
+	};
+	const access_data_submit = (node) => {
+		return enhance(node, ({ data }) => {});
+	};
+	const access_form_callback = (node) => {
+		return enhance(
+			node,
+			() =>
+				({ form, update }) =>
+					update()
+		);
+	};
+	const access_data_callback = (node) => {
+		return enhance(
+			node,
+			() =>
+				({ data, update }) =>
+					update()
+		);
+	};
+</script>
+
+<form method="POST" action="?/form_submit" use:access_form_submit>
+	<button id="access-form-in-submit" type="submit"
+		>{form?.form_submit ? 'processed' : 'not processed'}</button
+	>
+</form>
+
+<form method="POST" action="?/data_submit" use:access_data_submit>
+	<button id="access-data-in-submit" type="submit"
+		>{form?.data_submit ? 'processed' : 'not processed'}</button
+	>
+</form>
+
+<form method="POST" action="?/form_callback" use:access_form_callback>
+	<button id="access-form-in-callback" type="submit"
+		>{form?.form_callback ? 'processed' : 'not processed'}</button
+	>
+</form>
+
+<form method="POST" action="?/data_callback" use:access_data_callback>
+	<button id="access-data-in-callback" type="submit"
+		>{form?.data_callback ? 'processed' : 'not processed'}</button
+	>
+</form>

packages/kit/test/apps/basics/test/test.js

@@ -828,6 +828,51 @@ test.describe('Matchers', () => {
 });
 
 test.describe('Actions', () => {
+	for (const { id, old_name, new_name, call_location } of [
+		{
+			id: 'access-form-in-submit',
+			old_name: 'form',
+			new_name: 'formElement',
+			call_location: 'use:enhance submit function'
+		},
+		{
+			id: 'access-form-in-callback',
+			old_name: 'form',
+			new_name: 'formElement',
+			call_location: 'callback returned from use:enhance submit function'
+		},
+		{
+			id: 'access-data-in-submit',
+			old_name: 'data',
+			new_name: 'formData',
+			call_location: 'use:enhance submit function'
+		},
+		{
+			id: 'access-data-in-callback',
+			old_name: 'data',
+			new_name: 'formData',
+			call_location: 'callback returned from use:enhance submit function'
+		}
+	]) {
+		test(`Accessing v2 deprecated properties results in a warning log, type: ${id}`, async ({
+			page,
+			javaScriptEnabled
+		}) => {
+			test.skip(!javaScriptEnabled, 'skip when js is disabled');
+			test.skip(!process.env.DEV, 'skip when not in dev mode');
+			await page.goto('/actions/enhance/old-property-access');
+			const log_promise = page.waitForEvent('console');
+			const button = page.locator(`#${id}`);
+			await button.click();
+			expect(await button.textContent()).toBe('processed'); // needed to make sure action completes
+			const log = await log_promise;
+			expect(log.text()).toBe(
+				`\`${old_name}\` has been deprecated in favor of \`${new_name}\`. \`${old_name}\` will be removed in a future version. (Called from ${call_location})`
+			);
+			expect(log.type()).toBe('warning');
+		});
+	}
+
 	test('Error props are returned', async ({ page, javaScriptEnabled }) => {
 		await page.goto('/actions/form-errors');
 		await page.click('button');

packages/kit/test/apps/dev-only/package.json

@@ -1,5 +1,5 @@
 {
-	"name": "test-basics",
+	"name": "test-dev-only",
 	"private": true,
 	"version": "0.0.2-next.0",
 	"scripts": {

packages/kit/types/ambient.d.ts

@@ -80,15 +80,36 @@ declare module '$app/forms' {
 		Invalid extends Record<string, unknown> | undefined = Record<string, any>
 	> = (input: {
 		action: URL;
+		/**
+		 * use `formData` instead of `data`
+		 * @deprecated
+		 */
 		data: FormData;
+		formData: FormData;
+		/**
+		 * use `formElement` instead of `form`
+		 * @deprecated
+		 */
 		form: HTMLFormElement;
+		formElement: HTMLFormElement;
 		controller: AbortController;
 		cancel(): void;
 		submitter: HTMLElement | null;
 	}) => MaybePromise<
 		| void
 		| ((opts: {
+				/**
+				 * use `formData` instead of `data`
+				 * @deprecated
+				 */
+				data: FormData;
+				formData: FormData;
+				/**
+				 * use `formElement` instead of `form`
+				 * @deprecated
+				 */
 				form: HTMLFormElement;
+				formElement: HTMLFormElement;
 				action: URL;
 				result: ActionResult<Success, Invalid>;
 				/**
@@ -108,7 +129,7 @@ declare module '$app/forms' {
 		Success extends Record<string, unknown> | undefined = Record<string, any>,
 		Invalid extends Record<string, unknown> | undefined = Record<string, any>
 	>(
-		form: HTMLFormElement,
+		formElement: HTMLFormElement,
 		/**
 		 * Called upon submission with the given FormData and the `action` that should be triggered.
 		 * If `cancel` is called, the form will not be submitted.