Form actions

A +page.server.js file can export actions, which allow you to POST data to the server using the <form> element.

When using <form>, client-side JavaScript is optional, but you can easily progressively enhance your form interactions with JavaScript to provide the best user experience.

Default actions

In the simplest case, a page declares a default action:

/** @satisfies {import('./$types').Actions} */
export const actions = {
	default: async (event) => {
		// TODO log the user in
	}
};

To invoke this action from the /login page, just add a <form> — no JavaScript needed:

<form method="POST">
	<label>
		Email
		<input name="email" type="email">
	</label>
	<label>
		Password
		<input name="password" type="password">
	</label>
	<button>Log in</button>
</form>

If someone were to click the button, the browser would send the form data via POST request to the server, running the default action.

We can also invoke the action from other pages (for example if there's a login widget in the nav in the root layout) by adding the action attribute, pointing to the page:

<form method="POST" action="/login">
	<!-- content -->
</form>

Submitting this form takes you to /login, whether the action succeeds or fails — that's what the browser does, and use:enhance emulates it. If you'd rather stay where you are and handle the result in place, there's a recipe for that in the progressive enhancement section.

Named actions

Instead of one default action, a page can have as many named actions as it needs:

/** @satisfies {import('./$types').Actions} */
export const actions = {
	default: async (event) => {
	login: async (event) => {
		// TODO log the user in
	},
	register: async (event) => {
		// TODO register the user
	}
};

To invoke a named action, add a query parameter with the name prefixed by a / character:

<form method="POST" action="?/register">

<form method="POST" action="/login?/register">

As well as the action attribute, we can use the formaction attribute on a button to POST the same form data to a different action than the parent <form>:

<form method="POST" action="?/login">
	<label>
		Email
		<input name="email" type="email">
	</label>
	<label>
		Password
		<input name="password" type="password">
	</label>
	<button>Log in</button>
	<button formaction="?/register">Register</button>
</form>

Anatomy of an action

Each action receives a RequestEvent object, allowing you to read the data with request.formData(). After processing the request (for example, logging the user in by setting a cookie), the action can respond with data that will be available through the form property on the corresponding page and through page.form app-wide until the next update.

import * as db from '#lib/server/db.js';

/** @type {import('./$types').PageServerLoad} */
export async function load({ cookies }) {
	const user = await db.getUserFromSession(cookies.get('sessionid'));
	return { user };
}

/** @satisfies {import('./$types').Actions} */
export const actions = {
	login: async ({ cookies, request }) => {
		const data = await request.formData();
		const email = data.get('email');
		const password = data.get('password');

		const user = await db.getUser(email);
		cookies.set('sessionid', await db.createSession(user));

		return { success: true };
	},
	register: async (event) => {
		// TODO register the user
	}
};

<script>
	/** @type {import('./$types').PageProps} */
	let { data, form } = $props();
</script>

{#if form?.success}
	<!-- this message is ephemeral; it exists because the page was rendered in
	       response to a form submission. it will vanish if the user reloads -->
	<p>Successfully logged in! Welcome back, {data.user.name}</p>
{/if}

/** @type {{ data: import('./$types').PageData, form: import('./$types').ActionData }} */
let { data, form } = $props();

Validation errors

If the request couldn't be processed because of invalid data, you can return validation errors — along with the previously submitted form values — back to the user so that they can try again. The fail function lets you return an HTTP status code (typically 400 or 422, in the case of validation errors) along with the data. The status code is available through page.status and the data through form:

When using use:enhance (or making a fetch request with the accept: application/json header), the HTTP response status code will match the status code passed to fail. When an action returns data, the response will have status 200, and when it returns nothing (i.e. undefined), the response will have status 204. This makes it easier to track form submission outcomes using observability tools.

import { fail } from '@sveltejs/kit';
import * as db from '#lib/server/db.js';

/** @satisfies {import('./$types').Actions} */
export const actions = {
	login: async ({ cookies, request }) => {
		const data = await request.formData();
		const email = data.get('email');
		const password = data.get('password');

		if (!email) {
			return fail(400, { email, missing: true });
		}

		const user = await db.getUser(email);

		if (!user || user.password !== db.hash(password)) {
			return fail(400, { email, incorrect: true });
		}

		cookies.set('sessionid', await db.createSession(user));

		return { success: true };
	},
	register: async (event) => {
		// TODO register the user
	}
};

<form method="POST" action="?/login">
	{#if form?.missing}<p class="error">The email field is required</p>{/if}
	{#if form?.incorrect}<p class="error">Invalid credentials!</p>{/if}
	<label>
		Email
		<input name="email" type="email" value={form?.email ?? ''}>
	</label>
	<label>
		Password
		<input name="password" type="password">
	</label>
	<button>Log in</button>
	<button formaction="?/register">Register</button>
</form>

The returned data must be serializable as JSON. Beyond that, the structure is entirely up to you. For example, if you had multiple forms on the page, you could distinguish which <form> the returned form data referred to with an id property or similar.

Redirects

Redirects (and errors) work exactly the same as in load:

import { fail, redirect } from '@sveltejs/kit';
import * as db from '#lib/server/db.js';

/** @satisfies {import('./$types').Actions} */
export const actions = {
	login: async ({ cookies, request, url }) => {
		const data = await request.formData();
		const email = data.get('email');
		const password = data.get('password');

		const user = await db.getUser(email);
		if (!user) {
			return fail(400, { email, missing: true });
		}

		if (user.password !== db.hash(password)) {
			return fail(400, { email, incorrect: true });
		}

		cookies.set('sessionid', await db.createSession(user));

		if (url.searchParams.has('redirectTo')) {
			redirect(303, url.searchParams.get('redirectTo'));
		}

		return { success: true };
	},
	register: async (event) => {
		// TODO register the user
	}
};

Loading data

After an action runs, the page will be re-rendered (unless a redirect or an unexpected error occurs), with the action's return value available to the page as the form prop. This means that your page's load functions will run after the action completes.

Note that handle runs before the action is invoked, and does not rerun before the load functions. This means that if, for example, you use handle to populate event.locals based on a cookie, you must update event.locals when you set or delete the cookie in an action:

/** @type {import('@sveltejs/kit/hooks').Handle} */
export async function handle({ event, resolve }) {
	event.locals.user = await getUser(event.cookies.get('sessionid'));
	return resolve(event);
}

/** @type {import('./$types').PageServerLoad} */
export function load(event) {
	return {
		user: event.locals.user
	};
}

/** @satisfies {import('./$types').Actions} */
export const actions = {
	logout: async (event) => {
		event.cookies.delete('sessionid');
		event.locals.user = null;
	}
};

Progressive enhancement

In the preceding sections we built a /login action that works without client-side JavaScript — not a fetch in sight. That's great, but when JavaScript is available we can progressively enhance our form interactions to provide a better user experience.

use:enhance

The easiest way to progressively enhance a form is to add the use:enhance action:

<script>
	import { enhance } from '$app/forms';

	/** @type {import('./$types').PageProps} */
	let { form } = $props();
</script>

<form method="POST" use:enhance>

Without an argument, use:enhance will emulate the browser-native behaviour, just without the full-page reloads. It will:

• if the action is on the page you're submitting from, emulate a reload of the current page — updating the form property, page.form and page.status on a successful or invalid response
• if the action is on a different page (for example <form action="/somewhere/else" ..>), navigate to the action's URL on a successful or invalid response, populating that page's form property and page.status, just as a native submission would. A new history entry is pushed
• reset the <form> element
• refresh all data using refreshAll on a successful response
• call goto on a redirect response
• render the nearest +error boundary if an unexpected error occurs — the boundary nearest the action's route, if the action is on a different page
• reset focus to the appropriate element

If you don't want a cross-page submission to navigate — a common need for login or newsletter widgets that live in a layout and post to another page's action — pass navigate: false to update, which gives you the same behaviour as SvelteKit 2:

<form
	method="POST"
	action="/other/page?/subscribe"
	use:enhance={() => async ({ update }) => {
		await update({ navigate: false });
	}}
>

The result is applied to the page you're on: form, page.form and page.status are updated, and error results render the current route's nearest +error boundary.

Customising use:enhance

To customise the behaviour, you can provide a SubmitFunction that runs immediately before the form is submitted, and (optionally) returns a callback that runs with the ActionResult.

<form
	method="POST"
	use:enhance={({ formElement, formData, action, cancel, submitter }) => {
		// `formElement` is this `<form>` element
		// `formData` is its `FormData` object that's about to be submitted
		// `action` is the URL to which the form is posted
		// calling `cancel()` will prevent the submission
		// `submitter` is the `HTMLElement` that caused the form to be submitted

		return async ({ result, update }) => {
			// `result` is an `ActionResult` object
			// `update` is a function which triggers the default logic that would be triggered if this callback wasn't set
		};
	}}
>

You can use these functions to show and hide loading UI, and so on.

If you return a callback, you override the default post-submission behavior. To get it back, call update, which accepts navigate, refreshAll and reset parameters, or use applyAction on the result:

<script>
	import { enhance, applyAction } from '$app/forms';

	/** @type {import('./$types').PageProps} */
	let { form } = $props();
</script>

<form
	method="POST"
	use:enhance={({ formElement, formData, action, cancel }) => {
		return async ({ result }) => {
			// `result` is an `ActionResult` object
			if (result.type === 'redirect') {
				goto(result.location);
			} else {
				await applyAction(result);
			}
		};
	}}
>

The behaviour of applyAction(result) depends on result.type:

• success, failure — sets page.status to result.status and updates form and page.form to result.data
• redirect — calls goto(result.location, { refreshAll: true })
• error — renders the current route's nearest +error boundary with result.error

In all cases, focus will be reset. applyAction does not navigate to the location of a non-redirect result or refresh data. Use update to get the complete default enhanced behavior, including carrying form data and status into a cross-page navigation. When implementing enhancement manually, inspect result.location and use goto or refreshAll as appropriate.

Custom event listener

We can also implement progressive enhancement ourselves, without use:enhance, with a normal event listener on the <form>:

<script>
	import { refreshAll, goto } from '$app/navigation';
	import { applyAction, deserialize } from '$app/forms';

	/** @type {import('./$types').PageProps} */
	let { form } = $props();

	/** @param {SubmitEvent & { currentTarget: EventTarget & HTMLFormElement}} event */
	async function handleSubmit(event) {
		event.preventDefault();
		const data = new FormData(event.currentTarget, event.submitter);

		const response = await fetch(event.currentTarget.action, {
			method: 'POST',
			body: data
		});

		/** @type {import('$app/forms').ActionResult} */
		const result = deserialize(await response.text());

		if (result.type !== 'redirect' && result.location !== undefined) {
			if (result.location === location.pathname + location.search) {
				if (result.type === 'success') await refreshAll();
			} else {
				await goto(result.location, { refreshAll: result.type === 'success' });
			}
		}

		await applyAction(result);
	}
</script>

<form method="POST" onsubmit={handleSubmit}>
	<!-- content -->
</form>

Note that you need to deserialize the response before processing it further using the corresponding method from $app/forms. JSON.parse() isn't enough because form actions - like load functions - also support returning Date or BigInt objects.

If you have a +server.js alongside your +page.server.js, fetch requests will be routed there by default. To POST to an action in +page.server.js instead, use the custom x-sveltekit-action header:

const response = await fetch(this.action, {
	method: 'POST',
	body: data,
	headers: {
		'x-sveltekit-action': 'true'
	}
});

Alternatives

Form actions are the preferred way to send data to the server, since they can be progressively enhanced, but you can also use +server.js files to expose (for example) a JSON API. Here's how such an interaction could look like:

<script>
	function rerun() {
		fetch('/api/ci', {
			method: 'POST'
		});
	}
</script>

<button onclick={rerun}>Rerun CI</button>

/** @type {import('./$types').RequestHandler} */
export function POST() {
	// do something
}

GET vs POST

As we've seen, to invoke a form action you must use method="POST".

Some forms don't need to POST data to the server — search inputs, for example. For these you can use method="GET" (or, equivalently, no method at all), and SvelteKit will treat them like <a> elements, using the client-side router instead of a full page navigation:

<form action="/search">
	<label>
		Search
		<input name="q">
	</label>
</form>

Submitting this form will navigate to /search?q=... and invoke your load function but will not invoke an action. As with <a> elements, you can set the data-sveltekit-reload, data-sveltekit-replacestate and data-sveltekit-reset attributes on the <form> to control the router's behaviour.

Further reading

• Tutorial: Forms