SvelteKitCore concepts

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:

src/routes/login/+page.server

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

import type { type Actions = {
    [x: string]: Kit.Action<Record<string, any>, void | Record<string, any>, string | null>;
}
type Actions = {
    [x: string]: Kit.Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions } from './$types';

export const const actions: {
    default: (event: Kit.RequestEvent<Record<string, any>, string | null>) => Promise<void>;
}actions = {
	default: (event: Kit.RequestEvent<Record<string, any>, string | null>) => Promise<void>default: async (event: Kit.RequestEvent<Record<string, any>, string | null>event) => {
		// TODO log the user in
	}
} satisfies type Actions = {
    [x: string]: Kit.Action<Record<string, any>, void | Record<string, any>, string | null>;
}
type Actions = {
    [x: string]: Kit.Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions;

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

src/routes/login/+page

<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.

Actions always use POST requests, since GET requests should never have side-effects.

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:

src/routes/+layout

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

Named actions

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

src/routes/login/+page.server

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

import type { type Actions = {
    [x: string]: Kit.Action<Record<string, any>, void | Record<string, any>, string | null>;
}
type Actions = {
    [x: string]: Kit.Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions } from './$types';

export const const actions: {
    login: (event: Kit.RequestEvent<Record<string, any>, string | null>) => Promise<void>;
    register: (event: Kit.RequestEvent<Record<string, any>, string | null>) => Promise<...>;
}actions = {
	default: async (event) => {
	login: (event: Kit.RequestEvent<Record<string, any>, string | null>) => Promise<void>login: async (event: Kit.RequestEvent<Record<string, any>, string | null>event) => {
		// TODO log the user in
	},
	register: (event: Kit.RequestEvent<Record<string, any>, string | null>) => Promise<void>register: async (event: Kit.RequestEvent<Record<string, any>, string | null>event) => {
		// TODO register the user
	}
} satisfies type Actions = {
    [x: string]: Kit.Action<Record<string, any>, void | Record<string, any>, string | null>;
}
type Actions = {
    [x: string]: Kit.Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions;

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

src/routes/login/+page

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

src/routes/+layout

<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>:

src/routes/login/+page

<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>

We can’t have default actions next to named actions, because if you POST to a named action without a redirect, the query parameter is persisted in the URL, which means the next default POST would go through the named action from before.

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.

src/routes/login/+page.server

import * as module "$lib/server/db"db from '$lib/server/db';

/** @type {import('./$types').PageServerLoad} */
export async function function load(event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>): MaybePromise<void | Record<string, any>>@type{import('./$types').PageServerLoad}load({ cookies: CookiesGet or set cookies related to the current request
cookies }) {
	const const user: anyuser = await module "$lib/server/db"db.getUserFromSession(cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.get(name: string, opts?: CookieParseOptions): string | undefinedGets a cookie that was previously set with cookies.set, or from the request headers.
@paramname the name of the cookie
@paramopts the options, passed directly to cookie.parse. See documentation hereget('sessionid'));
	return { user: anyuser };
}

/** @satisfies {import('./$types').Actions} */
export const const actions: {
    login: ({ cookies, request }: RequestEvent<Record<string, any>, string | null>) => Promise<{
        success: boolean;
    }>;
    register: (event: RequestEvent<Record<string, any>, string | null>) => Promise<...>;
}@satisfies{import('./$types').Actions}actions = {
	login: ({ cookies, request }: RequestEvent<Record<string, any>, string | null>) => Promise<{
    success: boolean;
}>login: async ({ cookies: CookiesGet or set cookies related to the current request
cookies, request: RequestThe original request object
request }) => {
		const const data: FormDatadata = await request: RequestThe original request object
request.Body.formData(): Promise<FormData>MDN Reference
formData();
		const const email: FormDataEntryValue | nullemail = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('email');
		const const password: FormDataEntryValue | nullpassword = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('password');

		const const user: anyuser = await module "$lib/server/db"db.getUser(const email: FormDataEntryValue | nullemail);
		cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.set(name: string, value: string, opts: CookieSerializeOptions & {
    path: string;
}): voidSets a cookie. This will add a set-cookie header to the response, but also make the cookie available via cookies.get or cookies.getAll during the current request.
The httpOnly and secure options are true by default (except on http://localhost, where secure is false), and must be explicitly disabled if you want cookies to be readable by client-side JavaScript and/or transmitted over HTTP. The sameSite option defaults to lax.
You must specify a path for the cookie. In most cases you should explicitly set path: '/' to make the cookie available throughout your app. You can use relative paths, or set path: '' to make the cookie only available on the current path and its children
@paramname the name of the cookie
@paramvalue the cookie value
@paramopts the options, passed directly to cookie.serialize. See documentation hereset('sessionid', await module "$lib/server/db"db.createSession(const user: anyuser), { path: stringSpecifies the value for the 
{@link 
https://tools.ietf.org/html/rfc6265#section-5.2.4 Path Set-Cookie attribute
}
.
By default, the path is considered the “default path”.
path: '/' });

		return { success: booleansuccess: true };
	},
	register: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>register: async (event: RequestEvent<Record<string, any>, string | null>event) => {
		// TODO register the user
	}
};

import * as module "$lib/server/db"db from '$lib/server/db';
import type { type PageServerLoad = (event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>PageServerLoad, type Actions = {
    [x: string]: Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions } from './$types';

export const const load: PageServerLoadload: type PageServerLoad = (event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>PageServerLoad = async ({ cookies: CookiesGet or set cookies related to the current request
cookies }) => {
	const const user: anyuser = await module "$lib/server/db"db.getUserFromSession(cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.get(name: string, opts?: CookieParseOptions): string | undefinedGets a cookie that was previously set with cookies.set, or from the request headers.
@paramname the name of the cookie
@paramopts the options, passed directly to cookie.parse. See documentation hereget('sessionid'));
	return { user: anyuser };
};

export const const actions: {
    login: ({ cookies, request }: RequestEvent<Record<string, any>, string | null>) => Promise<{
        success: boolean;
    }>;
    register: (event: RequestEvent<Record<string, any>, string | null>) => Promise<...>;
}actions = {
	login: ({ cookies, request }: RequestEvent<Record<string, any>, string | null>) => Promise<{
    success: boolean;
}>login: async ({ cookies: CookiesGet or set cookies related to the current request
cookies, request: RequestThe original request object
request }) => {
		const const data: FormDatadata = await request: RequestThe original request object
request.Body.formData(): Promise<FormData>MDN Reference
formData();
		const const email: FormDataEntryValue | nullemail = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('email');
		const const password: FormDataEntryValue | nullpassword = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('password');

		const const user: anyuser = await module "$lib/server/db"db.getUser(const email: FormDataEntryValue | nullemail);
		cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.set(name: string, value: string, opts: CookieSerializeOptions & {
    path: string;
}): voidSets a cookie. This will add a set-cookie header to the response, but also make the cookie available via cookies.get or cookies.getAll during the current request.
The httpOnly and secure options are true by default (except on http://localhost, where secure is false), and must be explicitly disabled if you want cookies to be readable by client-side JavaScript and/or transmitted over HTTP. The sameSite option defaults to lax.
You must specify a path for the cookie. In most cases you should explicitly set path: '/' to make the cookie available throughout your app. You can use relative paths, or set path: '' to make the cookie only available on the current path and its children
@paramname the name of the cookie
@paramvalue the cookie value
@paramopts the options, passed directly to cookie.serialize. See documentation hereset('sessionid', await module "$lib/server/db"db.createSession(const user: anyuser), { path: stringSpecifies the value for the 
{@link 
https://tools.ietf.org/html/rfc6265#section-5.2.4 Path Set-Cookie attribute
}
.
By default, the path is considered the “default path”.
path: '/' });

		return { success: booleansuccess: true };
	},
	register: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>register: async (event: RequestEvent<Record<string, any>, string | null>event) => {
		// TODO register the user
	}
} satisfies type Actions = {
    [x: string]: Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions;

src/routes/login/+page

<script>
	/** @type {import('./$types').PageData} */
	export let data;

	/** @type {import('./$types').ActionData} */
	export let form;
</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}

<script lang="ts">
	import type { PageData, ActionData } from './$types';

	export let data: PageData;

	export let form: ActionData;
</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}

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:

src/routes/login/+page.server

import { function fail(status: number): ActionFailure<undefined> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.fail } from '@sveltejs/kit';
import * as module "$lib/server/db"db from '$lib/server/db';

/** @satisfies {import('./$types').Actions} */
export const const actions: {
    login: ({ cookies, request }: RequestEvent<Record<string, any>, string | null>) => Promise<ActionFailure<{
        email: string | null;
        missing: boolean;
    }> | ActionFailure<{
        ...;
    }> | {
        ...;
    }>;
    register: (event: RequestEvent<...>) => Promise<...>;
}@satisfies{import('./$types').Actions}actions = {
	login: ({ cookies, request }: RequestEvent<Record<string, any>, string | null>) => Promise<ActionFailure<{
    email: string | null;
    missing: boolean;
}> | ActionFailure<{
    email: FormDataEntryValue;
    incorrect: boolean;
}> | {
    ...;
}>login: async ({ cookies: CookiesGet or set cookies related to the current request
cookies, request: RequestThe original request object
request }) => {
		const const data: FormDatadata = await request: RequestThe original request object
request.Body.formData(): Promise<FormData>MDN Reference
formData();
		const const email: FormDataEntryValue | nullemail = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('email');
		const const password: FormDataEntryValue | nullpassword = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('password');

		if (!const email: FormDataEntryValue | nullemail) {
			return fail<{
    email: string | null;
    missing: boolean;
}>(status: number, data: {
    email: string | null;
    missing: boolean;
}): ActionFailure<{
    email: string | null;
    missing: boolean;
}> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.
@paramdata Data associated with the failure (e.g. validation errors)fail(400, { email: string | nullemail, missing: booleanmissing: true });
		}

		const const user: anyuser = await module "$lib/server/db"db.getUser(const email: FormDataEntryValueemail);

		if (!const user: anyuser || const user: anyuser.password !== module "$lib/server/db"db.hash(const password: FormDataEntryValue | nullpassword)) {
			return fail<{
    email: FormDataEntryValue;
    incorrect: boolean;
}>(status: number, data: {
    email: FormDataEntryValue;
    incorrect: boolean;
}): ActionFailure<{
    email: FormDataEntryValue;
    incorrect: boolean;
}> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.
@paramdata Data associated with the failure (e.g. validation errors)fail(400, { email: FormDataEntryValueemail, incorrect: booleanincorrect: true });
		}

		cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.set(name: string, value: string, opts: CookieSerializeOptions & {
    path: string;
}): voidSets a cookie. This will add a set-cookie header to the response, but also make the cookie available via cookies.get or cookies.getAll during the current request.
The httpOnly and secure options are true by default (except on http://localhost, where secure is false), and must be explicitly disabled if you want cookies to be readable by client-side JavaScript and/or transmitted over HTTP. The sameSite option defaults to lax.
You must specify a path for the cookie. In most cases you should explicitly set path: '/' to make the cookie available throughout your app. You can use relative paths, or set path: '' to make the cookie only available on the current path and its children
@paramname the name of the cookie
@paramvalue the cookie value
@paramopts the options, passed directly to cookie.serialize. See documentation hereset('sessionid', await module "$lib/server/db"db.createSession(const user: anyuser), { path: stringSpecifies the value for the 
{@link 
https://tools.ietf.org/html/rfc6265#section-5.2.4 Path Set-Cookie attribute
}
.
By default, the path is considered the “default path”.
path: '/' });

		return { success: booleansuccess: true };
	},
	register: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>register: async (event: RequestEvent<Record<string, any>, string | null>event) => {
		// TODO register the user
	}
};

import { function fail(status: number): ActionFailure<undefined> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.fail } from '@sveltejs/kit';
import * as module "$lib/server/db"db from '$lib/server/db';
import type { type Actions = {
    [x: string]: Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions } from './$types';

export const const actions: {
    login: ({ cookies, request }: RequestEvent<Record<string, any>, string | null>) => Promise<ActionFailure<{
        email: string | null;
        missing: boolean;
    }> | ActionFailure<{
        ...;
    }> | {
        ...;
    }>;
    register: (event: RequestEvent<...>) => Promise<...>;
}actions = {
	login: ({ cookies, request }: RequestEvent<Record<string, any>, string | null>) => Promise<ActionFailure<{
    email: string | null;
    missing: boolean;
}> | ActionFailure<{
    email: FormDataEntryValue;
    incorrect: boolean;
}> | {
    ...;
}>login: async ({ cookies: CookiesGet or set cookies related to the current request
cookies, request: RequestThe original request object
request }) => {
		const const data: FormDatadata = await request: RequestThe original request object
request.Body.formData(): Promise<FormData>MDN Reference
formData();
		const const email: FormDataEntryValue | nullemail = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('email');
		const const password: FormDataEntryValue | nullpassword = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('password');

		if (!const email: FormDataEntryValue | nullemail) {
			return fail<{
    email: string | null;
    missing: boolean;
}>(status: number, data: {
    email: string | null;
    missing: boolean;
}): ActionFailure<{
    email: string | null;
    missing: boolean;
}> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.
@paramdata Data associated with the failure (e.g. validation errors)fail(400, { email: string | nullemail, missing: booleanmissing: true });
		}

		const const user: anyuser = await module "$lib/server/db"db.getUser(const email: FormDataEntryValueemail);

		if (!const user: anyuser || const user: anyuser.password !== module "$lib/server/db"db.hash(const password: FormDataEntryValue | nullpassword)) {
			return fail<{
    email: FormDataEntryValue;
    incorrect: boolean;
}>(status: number, data: {
    email: FormDataEntryValue;
    incorrect: boolean;
}): ActionFailure<{
    email: FormDataEntryValue;
    incorrect: boolean;
}> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.
@paramdata Data associated with the failure (e.g. validation errors)fail(400, { email: FormDataEntryValueemail, incorrect: booleanincorrect: true });
		}

		cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.set(name: string, value: string, opts: CookieSerializeOptions & {
    path: string;
}): voidSets a cookie. This will add a set-cookie header to the response, but also make the cookie available via cookies.get or cookies.getAll during the current request.
The httpOnly and secure options are true by default (except on http://localhost, where secure is false), and must be explicitly disabled if you want cookies to be readable by client-side JavaScript and/or transmitted over HTTP. The sameSite option defaults to lax.
You must specify a path for the cookie. In most cases you should explicitly set path: '/' to make the cookie available throughout your app. You can use relative paths, or set path: '' to make the cookie only available on the current path and its children
@paramname the name of the cookie
@paramvalue the cookie value
@paramopts the options, passed directly to cookie.serialize. See documentation hereset('sessionid', await module "$lib/server/db"db.createSession(const user: anyuser), { path: stringSpecifies the value for the 
{@link 
https://tools.ietf.org/html/rfc6265#section-5.2.4 Path Set-Cookie attribute
}
.
By default, the path is considered the “default path”.
path: '/' });

		return { success: booleansuccess: true };
	},
	register: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>register: async (event: RequestEvent<Record<string, any>, string | null>event) => {
		// TODO register the user
	}
} satisfies type Actions = {
    [x: string]: Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions;

Note that as a precaution, we only return the email back to the page — not the password.

src/routes/login/+page

<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:

src/routes/login/+page.server

import { function fail(status: number): ActionFailure<undefined> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.fail, function redirect(status: 300 | 301 | 302 | 303 | 304 | 305 | 306 | 307 | 308 | ({} & number), location: string | URL): neverRedirect a request. When called during request handling, SvelteKit will return a redirect response.
Make sure you’re not catching the thrown redirect, which would prevent SvelteKit from handling it.
@paramstatus The HTTP status code. Must be in the range 300-308.
@paramlocation The location to redirect to.
@throwsRedirect This error instructs SvelteKit to redirect to the specified location.
@throwsError If the provided status is invalid.redirect } from '@sveltejs/kit';
import * as module "$lib/server/db"db from '$lib/server/db';

/** @satisfies {import('./$types').Actions} */
export const const actions: {
    login: ({ cookies, request, url }: RequestEvent<Record<string, any>, string | null>) => Promise<ActionFailure<{
        email: FormDataEntryValue | null;
        missing: boolean;
    }> | ActionFailure<...> | {
        ...;
    }>;
    register: (event: RequestEvent<...>) => Promise<...>;
}@satisfies{import('./$types').Actions}actions = {
	login: ({ cookies, request, url }: RequestEvent<Record<string, any>, string | null>) => Promise<ActionFailure<{
    email: FormDataEntryValue | null;
    missing: boolean;
}> | ActionFailure<...> | {
    ...;
}>login: async ({ cookies: CookiesGet or set cookies related to the current request
cookies, request: RequestThe original request object
request, url: URLThe requested URL.
url }) => {
		const const data: FormDatadata = await request: RequestThe original request object
request.Body.formData(): Promise<FormData>MDN Reference
formData();
		const const email: FormDataEntryValue | nullemail = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('email');
		const const password: FormDataEntryValue | nullpassword = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('password');

		const const user: anyuser = await module "$lib/server/db"db.getUser(const email: FormDataEntryValue | nullemail);
		if (!const user: anyuser) {
			return fail<{
    email: FormDataEntryValue | null;
    missing: boolean;
}>(status: number, data: {
    email: FormDataEntryValue | null;
    missing: boolean;
}): ActionFailure<...> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.
@paramdata Data associated with the failure (e.g. validation errors)fail(400, { email: FormDataEntryValue | nullemail, missing: booleanmissing: true });
		}

		if (const user: anyuser.password !== module "$lib/server/db"db.hash(const password: FormDataEntryValue | nullpassword)) {
			return fail<{
    email: FormDataEntryValue | null;
    incorrect: boolean;
}>(status: number, data: {
    email: FormDataEntryValue | null;
    incorrect: boolean;
}): ActionFailure<...> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.
@paramdata Data associated with the failure (e.g. validation errors)fail(400, { email: FormDataEntryValue | nullemail, incorrect: booleanincorrect: true });
		}

		cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.set(name: string, value: string, opts: CookieSerializeOptions & {
    path: string;
}): voidSets a cookie. This will add a set-cookie header to the response, but also make the cookie available via cookies.get or cookies.getAll during the current request.
The httpOnly and secure options are true by default (except on http://localhost, where secure is false), and must be explicitly disabled if you want cookies to be readable by client-side JavaScript and/or transmitted over HTTP. The sameSite option defaults to lax.
You must specify a path for the cookie. In most cases you should explicitly set path: '/' to make the cookie available throughout your app. You can use relative paths, or set path: '' to make the cookie only available on the current path and its children
@paramname the name of the cookie
@paramvalue the cookie value
@paramopts the options, passed directly to cookie.serialize. See documentation hereset('sessionid', await module "$lib/server/db"db.createSession(const user: anyuser), { path: stringSpecifies the value for the 
{@link 
https://tools.ietf.org/html/rfc6265#section-5.2.4 Path Set-Cookie attribute
}
.
By default, the path is considered the “default path”.
path: '/' });

		if (url: URLThe requested URL.
url.URL.searchParams: URLSearchParamsMDN Reference
searchParams.URLSearchParams.has(name: string, value?: string): booleanReturns a Boolean indicating if such a search parameter exists.
MDN Reference
has('redirectTo')) {
			function redirect(status: 300 | 301 | 302 | 303 | 304 | 305 | 306 | 307 | 308 | ({} & number), location: string | URL): neverRedirect a request. When called during request handling, SvelteKit will return a redirect response.
Make sure you’re not catching the thrown redirect, which would prevent SvelteKit from handling it.
@paramstatus The HTTP status code. Must be in the range 300-308.
@paramlocation The location to redirect to.
@throwsRedirect This error instructs SvelteKit to redirect to the specified location.
@throwsError If the provided status is invalid.redirect(303, url: URLThe requested URL.
url.URL.searchParams: URLSearchParamsMDN Reference
searchParams.URLSearchParams.get(name: string): string | nullReturns the first value associated to the given search parameter.
MDN Reference
get('redirectTo'));
		}

		return { success: booleansuccess: true };
	},
	register: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>register: async (event: RequestEvent<Record<string, any>, string | null>event) => {
		// TODO register the user
	}
};

import { function fail(status: number): ActionFailure<undefined> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.fail, function redirect(status: 300 | 301 | 302 | 303 | 304 | 305 | 306 | 307 | 308 | ({} & number), location: string | URL): neverRedirect a request. When called during request handling, SvelteKit will return a redirect response.
Make sure you’re not catching the thrown redirect, which would prevent SvelteKit from handling it.
@paramstatus The HTTP status code. Must be in the range 300-308.
@paramlocation The location to redirect to.
@throwsRedirect This error instructs SvelteKit to redirect to the specified location.
@throwsError If the provided status is invalid.redirect } from '@sveltejs/kit';
import * as module "$lib/server/db"db from '$lib/server/db';
import type { type Actions = {
    [x: string]: Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions } from './$types';

export const const actions: {
    login: ({ cookies, request, url }: RequestEvent<Record<string, any>, string | null>) => Promise<ActionFailure<{
        email: FormDataEntryValue | null;
        missing: boolean;
    }> | ActionFailure<...> | {
        ...;
    }>;
    register: (event: RequestEvent<...>) => Promise<...>;
}actions = {
	login: ({ cookies, request, url }: RequestEvent<Record<string, any>, string | null>) => Promise<ActionFailure<{
    email: FormDataEntryValue | null;
    missing: boolean;
}> | ActionFailure<...> | {
    ...;
}>login: async ({ cookies: CookiesGet or set cookies related to the current request
cookies, request: RequestThe original request object
request, url: URLThe requested URL.
url }) => {
		const const data: FormDatadata = await request: RequestThe original request object
request.Body.formData(): Promise<FormData>MDN Reference
formData();
		const const email: FormDataEntryValue | nullemail = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('email');
		const const password: FormDataEntryValue | nullpassword = const data: FormDatadata.FormData.get(name: string): FormDataEntryValue | nullMDN Reference
get('password');

		const const user: anyuser = await module "$lib/server/db"db.getUser(const email: FormDataEntryValue | nullemail);
		if (!const user: anyuser) {
			return fail<{
    email: FormDataEntryValue | null;
    missing: boolean;
}>(status: number, data: {
    email: FormDataEntryValue | null;
    missing: boolean;
}): ActionFailure<...> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.
@paramdata Data associated with the failure (e.g. validation errors)fail(400, { email: FormDataEntryValue | nullemail, missing: booleanmissing: true });
		}

		if (const user: anyuser.password !== module "$lib/server/db"db.hash(const password: FormDataEntryValue | nullpassword)) {
			return fail<{
    email: FormDataEntryValue | null;
    incorrect: boolean;
}>(status: number, data: {
    email: FormDataEntryValue | null;
    incorrect: boolean;
}): ActionFailure<...> (+1 overload)Create an ActionFailure object.
@paramstatus The HTTP status code. Must be in the range 400-599.
@paramdata Data associated with the failure (e.g. validation errors)fail(400, { email: FormDataEntryValue | nullemail, incorrect: booleanincorrect: true });
		}

		cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.set(name: string, value: string, opts: CookieSerializeOptions & {
    path: string;
}): voidSets a cookie. This will add a set-cookie header to the response, but also make the cookie available via cookies.get or cookies.getAll during the current request.
The httpOnly and secure options are true by default (except on http://localhost, where secure is false), and must be explicitly disabled if you want cookies to be readable by client-side JavaScript and/or transmitted over HTTP. The sameSite option defaults to lax.
You must specify a path for the cookie. In most cases you should explicitly set path: '/' to make the cookie available throughout your app. You can use relative paths, or set path: '' to make the cookie only available on the current path and its children
@paramname the name of the cookie
@paramvalue the cookie value
@paramopts the options, passed directly to cookie.serialize. See documentation hereset('sessionid', await module "$lib/server/db"db.createSession(const user: anyuser), { path: stringSpecifies the value for the 
{@link 
https://tools.ietf.org/html/rfc6265#section-5.2.4 Path Set-Cookie attribute
}
.
By default, the path is considered the “default path”.
path: '/' });

		if (url: URLThe requested URL.
url.URL.searchParams: URLSearchParamsMDN Reference
searchParams.URLSearchParams.has(name: string, value?: string): booleanReturns a Boolean indicating if such a search parameter exists.
MDN Reference
has('redirectTo')) {
			function redirect(status: 300 | 301 | 302 | 303 | 304 | 305 | 306 | 307 | 308 | ({} & number), location: string | URL): neverRedirect a request. When called during request handling, SvelteKit will return a redirect response.
Make sure you’re not catching the thrown redirect, which would prevent SvelteKit from handling it.
@paramstatus The HTTP status code. Must be in the range 300-308.
@paramlocation The location to redirect to.
@throwsRedirect This error instructs SvelteKit to redirect to the specified location.
@throwsError If the provided status is invalid.redirect(303, url.searchParams.get('redirectTo'));
		}

		return { success: booleansuccess: true };
	},
	register: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>register: async (event: RequestEvent<Record<string, any>, string | null>event) => {
		// TODO register the user
	}
} satisfies type Actions = {
    [x: string]: Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions;

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:

src/hooks.server

/** @type {import('@sveltejs/kit').Handle} */
export async function function handle(input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}): MaybePromise<...>@type{import('@sveltejs/kit').Handle}handle({ event: RequestEvent<Partial<Record<string, string>>, string | null>event, resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve }) {
	event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.locals: App.LocalsContains custom data that was added to the request within the handle hook.
locals.App.Locals.user: {
    name: string;
} | nulluser = await function getUser(sessionid: string | undefined): {
    name: string;
}getUser(event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.get(name: string, opts?: CookieParseOptions): string | undefinedGets a cookie that was previously set with cookies.set, or from the request headers.
@paramname the name of the cookie
@paramopts the options, passed directly to cookie.parse. See documentation hereget('sessionid'));
	return resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve(event: RequestEvent<Partial<Record<string, string>>, string | null>event);
}

import type { type Handle = (input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}) => MaybePromise<...>The handle hook runs every time the SvelteKit server receives a request and
determines the response.
It receives an event object representing the request and a function called resolve, which renders the route and generates a Response.
This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
Handle } from '@sveltejs/kit';

export const const handle: Handlehandle: type Handle = (input: {
    event: RequestEvent;
    resolve(event: RequestEvent, opts?: ResolveOptions): MaybePromise<Response>;
}) => MaybePromise<...>The handle hook runs every time the SvelteKit server receives a request and
determines the response.
It receives an event object representing the request and a function called resolve, which renders the route and generates a Response.
This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
Handle = async ({ event: RequestEvent<Partial<Record<string, string>>, string | null>event, resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve }) => {
	event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.locals: App.LocalsContains custom data that was added to the request within the handle hook.
locals.App.Locals.user: {
    name: string;
} | nulluser = await function getUser(sessionid: string | undefined): {
    name: string;
}getUser(event: RequestEvent<Partial<Record<string, string>>, string | null>event.RequestEvent<Partial<Record<string, string>>, string | null>.cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.get(name: string, opts?: CookieParseOptions): string | undefinedGets a cookie that was previously set with cookies.set, or from the request headers.
@paramname the name of the cookie
@paramopts the options, passed directly to cookie.parse. See documentation hereget('sessionid'));
	return resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve(event: RequestEvent<Partial<Record<string, string>>, string | null>event);
};

src/routes/account/+page.server

/** @type {import('./$types').PageServerLoad} */
export function function load(event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>): MaybePromise<void | Record<string, any>>@type{import('./$types').PageServerLoad}load(event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>event) {
	return {
		user: {
    name: string;
} | nulluser: event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>event.RequestEvent<Record<string, any>, string | null>.locals: App.LocalsContains custom data that was added to the request within the handle hook.
locals.App.Locals.user: {
    name: string;
} | nulluser
	};
}

/** @satisfies {import('./$types').Actions} */
export const const actions: {
    logout: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>;
}@satisfies{import('./$types').Actions}actions = {
	logout: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>logout: async (event: RequestEvent<Record<string, any>, string | null>event) => {
		event: RequestEvent<Record<string, any>, string | null>event.RequestEvent<Record<string, any>, string | null>.cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.delete(name: string, opts: CookieSerializeOptions & {
    path: string;
}): voidDeletes a cookie by setting its value to an empty string and setting the expiry date in the past.
You must specify a path for the cookie. In most cases you should explicitly set path: '/' to make the cookie available throughout your app. You can use relative paths, or set path: '' to make the cookie only available on the current path and its children
@paramname the name of the cookie
@paramopts the options, passed directly to cookie.serialize. The path must match the path of the cookie you want to delete. See documentation heredelete('sessionid', { path: stringSpecifies the value for the 
{@link 
https://tools.ietf.org/html/rfc6265#section-5.2.4 Path Set-Cookie attribute
}
.
By default, the path is considered the “default path”.
path: '/' });
		event: RequestEvent<Record<string, any>, string | null>event.RequestEvent<Params extends Partial<Record<string, string>> = Partial<Record<string, string>>, RouteId extends string | null = string | null>.locals: App.LocalsContains custom data that was added to the request within the handle hook.
locals.App.Locals.user: {
    name: string;
} | nulluser = null;
	}
};

import type { type PageServerLoad = (event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>PageServerLoad, type Actions = {
    [x: string]: Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions } from './$types';

export const const load: PageServerLoadload: type PageServerLoad = (event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>PageServerLoad = (event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>event) => {
	return {
		user: {
    name: string;
} | nulluser: event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>event.RequestEvent<Record<string, any>, string | null>.locals: App.LocalsContains custom data that was added to the request within the handle hook.
locals.App.Locals.user: {
    name: string;
} | nulluser
	};
};

export const const actions: {
    logout: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>;
}actions = {
	logout: (event: RequestEvent<Record<string, any>, string | null>) => Promise<void>logout: async (event: RequestEvent<Record<string, any>, string | null>event) => {
		event: RequestEvent<Record<string, any>, string | null>event.RequestEvent<Record<string, any>, string | null>.cookies: CookiesGet or set cookies related to the current request
cookies.Cookies.delete(name: string, opts: CookieSerializeOptions & {
    path: string;
}): voidDeletes a cookie by setting its value to an empty string and setting the expiry date in the past.
You must specify a path for the cookie. In most cases you should explicitly set path: '/' to make the cookie available throughout your app. You can use relative paths, or set path: '' to make the cookie only available on the current path and its children
@paramname the name of the cookie
@paramopts the options, passed directly to cookie.serialize. The path must match the path of the cookie you want to delete. See documentation heredelete('sessionid', { path: stringSpecifies the value for the 
{@link 
https://tools.ietf.org/html/rfc6265#section-5.2.4 Path Set-Cookie attribute
}
.
By default, the path is considered the “default path”.
path: '/' });
		event: RequestEvent<Record<string, any>, string | null>event.RequestEvent<Params extends Partial<Record<string, string>> = Partial<Record<string, string>>, RouteId extends string | null = string | null>.locals: App.LocalsContains custom data that was added to the request within the handle hook.
locals.App.Locals.user: {
    name: string;
} | nulluser = null;
	}
} satisfies type Actions = {
    [x: string]: Action<Record<string, any>, void | Record<string, any>, string | null>;
}Actions;

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:

src/routes/login/+page

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

	/** @type {import('./$types').ActionData} */
	export let form;
</script>

<form method="POST" use:enhance>

<script lang="ts">
	import { enhance } from '$app/forms';
	import type { ActionData } from './$types';

	export let form: ActionData;
</script>

<form method="POST" use:enhance>

use:enhance can only be used with forms that have method="POST". It will not work with method="GET", which is the default for forms without a specified method. Attempting to use use:enhance on forms without method="POST" will result in an error.

Yes, it’s a little confusing that the enhance action and <form action> are both called ‘action’. These docs are action-packed. Sorry.

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

update the form property, $page.form and $page.status on a successful or invalid response, but only if the action is on the same page you’re submitting from. For example, if your form looks like <form action="/somewhere/else" ..>, form and $page will not be updated. This is because in the native form submission case you would be redirected to the page the action is on. If you want to have them updated either way, use applyAction
reset the <form> element
invalidate all data using invalidateAll on a successful response
call goto on a redirect response
render the nearest +error boundary if an error occurs
reset focus to the appropriate element

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. Note that if you return a callback, the default behavior mentioned above is not triggered. To get it back, call update.

<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 may need to reproduce part of the default use:enhance behaviour, but without invalidating all data on a successful response. You can do so with applyAction:

src/routes/login/+page

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

	/** @type {import('./$types').ActionData} */
	export let form;
</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);
			}
		};
	}}
>

<script lang="ts">
	import { enhance, applyAction } from '$app/forms';
	import type { ActionData } from './$types';

	export let form: ActionData;
</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 (regardless of where you are submitting from, in contrast to update from enhance)
redirect — calls goto(result.location, { invalidateAll: true })
error — renders the nearest +error boundary with result.error

In all cases, focus will be reset.

Custom event listener

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

src/routes/login/+page

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

	/** @type {import('./$types').ActionData} */
	export let form;

	/** @type {any} */
	let error;

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

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

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

		if (result.type === 'success') {
			// rerun all `load` functions, following the successful update
			await invalidateAll();
		}

		applyAction(result);
	}
</script>

<form method="POST" on:submit|preventDefault={handleSubmit}>
	<!-- content -->
</form>

<script lang="ts">
	import { invalidateAll, goto } from '$app/navigation';
	import { applyAction, deserialize } from '$app/forms';
	import type { ActionData } from './$types';
	import type { ActionResult } from '@sveltejs/kit';

	export let form: ActionData;

	let error: any;

	async function handleSubmit(event: { currentTarget: EventTarget & HTMLFormElement}) {
		const data = new FormData(event.currentTarget);

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

		const result: ActionResult = deserialize(await response.text());

		if (result.type === 'success') {
			// rerun all `load` functions, following the successful update
			await invalidateAll();
		}

		applyAction(result);
	}
</script>

<form method="POST" on:submit|preventDefault={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 const response: Responseresponse = await function fetch(input: string | URL | globalThis.Request, init?: RequestInit): Promise<Response> (+1 overload)MDN Reference
fetch(this.action, {
	RequestInit.method?: string | undefinedA string to set request’s method.
method: 'POST',
	RequestInit.body?: BodyInit | null | undefinedA BodyInit object or null to set request’s body.
body: data,
	RequestInit.headers?: HeadersInit | undefinedA Headers object, an object literal, or an array of two-item arrays to set request’s headers.
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:

src/routes/send-message/+page

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

<button on:click={rerun}>Rerun CI</button>

src/routes/api/ci/+server

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

import type { type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>RequestHandler } from './$types';
export const const POST: RequestHandlerPOST: type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>RequestHandler = () => {
	// 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, data-sveltekit-keepfocus and data-sveltekit-noscroll attributes on the <form> to control the router’s behaviour.

Default actions

Named actions

Anatomy of an action

Validation errors

Redirects

Loading data

Progressive enhancement

use:enhance

Customising use:enhance

Custom event listener

Alternatives

GET vs POST

Further reading