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Form Actions

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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.js
ts
/** @type {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:

src/routes/login/+page.svelte
<form method="POST">
  <input name="email" type="email">
  <input name="password" type="password">
  <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.svelte
<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.js

/** @type {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:

src/routes/login/+page.svelte
<form method="POST" action="?/register">
src/routes/+layout.svelte
<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.svelte
<form method="POST">
<form method="POST" action="?/login">
  <input name="email" type="email">
  <input name="password" type="password">
  <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.js
ts
/** @type {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
  }
};
src/routes/login/+page.svelte
<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}

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 invalid function lets you return an HTTP status code (typically 400, in the case of validation errors) along with the data:

src/routes/login/+page.server.js
// @errors: 2339 2304
import { invalid } from '@sveltejs/kit';

/** @type {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 invalid(400, { email, missing: true });
		}

    const user = await db.getUser(email);

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

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

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

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

src/routes/login/+page.svelte
<form method="POST" action="?/login">
	<input name="email" type="email">
	{#if form?.missing}<p class="error">The email field is required</p>{/if}
	{#if form?.incorrect}<p class="error">Invalid credentials!</p>{/if}
	<input name="email" type="email" value={form?.email ?? ''}>

  <input name="password" type="password">
  <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.js
// @errors: 2339 2304
import { invalid, redirect } from '@sveltejs/kit';

/** @type {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 invalid(400, { email, missing: true });
    }

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

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

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

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

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

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

<form method="POST" use:enhance>

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 and $page.form and invalidate all data on a successful response
  • update the form property and $page.form on a invalid response
  • update $page.status on a successful or invalid response
  • call goto on a redirect response
  • render the nearest +error boundary if an error occurs

By default the form property and $page.form is only updated for actions that are in a +page.server.js alongside the +page.svelte because in the native form submission case you would be redirected to the page the action is on

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

<form
  method="POST"
  use:enhance={({ form, data, cancel }) => {
    // `form` is the `<form>` element    // `data` is its `FormData` object    // `cancel()` will prevent the submission
    return async ({ result }) => {
      // `result` is an `ActionResult` object    };
  }}
>

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

applyAction

If you provide your own callbacks, you may need to reproduce part of the default use:enhance behaviour, such as showing the nearest +error boundary. We can do this with applyAction:

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

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

<form
  method="POST"
  use:enhance={({ form, data, cancel }) => {
    // `form` is the `<form>` element
    // `data` is its `FormData` object
    // `cancel()` will prevent the submission

    return async ({ result }) => {
      // `result` is an `ActionResult` object
			if (result.type === 'error') {
				await applyAction(result);
			}
    };
  }}
>

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

  • success, invalid — sets $page.status to result.status and updates form and $page.form to result.data
  • redirect — calls goto(result.location)
  • error — renders the nearest +error boundary with result.error

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.svelte
<script>
  import { invalidateAll, goto } from '$app/navigation';
  import { applyAction } from '$app/forms';
  /** @type {import('./$types').ActionData} */  export let form;
  /** @type {any} */  let error;

  async function handleSubmit(event) {
    const data = new FormData(this);

    const response = await fetch(this.action, {
      method: 'POST',
      body: data
    });
    /** @type {import('@sveltejs/kit').ActionResult} */    const result = await response.json();

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

    applyAction(result);
  }
</script>

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

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.

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