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Basic SvelteKit
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Conclusion
Basic SvelteKit Forms Customizing use:enhance

With use:enhance, we can go further than just emulating the browser’s native behaviour. By providing a callback, we can add things like pending states and optimistic UI. Let’s simulate a slow network by adding an artificial delay to our two actions:

src/routes/+page.server
export const actions = {
	create: async ({ cookies, request }) => {
		await new Promise((fulfil) => setTimeout(fulfil, 1000));
		...
	},

	delete: async ({ cookies, request }) => {
		await new Promise((fulfil) => setTimeout(fulfil, 1000));
		...
	}
};

When we create or delete items, it now takes a full second before the UI updates, leaving the user wondering if they messed up somehow. To solve that, add some local state...

src/routes/+page
<script>
	import { fly, slide } from 'svelte/transition';
	import { enhance } from '$app/forms';

	let { data, form } = $props();

	let creating = $state(false);
	let deleting = $state([]);
</script>
<script lang="ts">
	import { fly, slide } from 'svelte/transition';
	import { enhance } from '$app/forms';

	let { data, form } = $props();

	let creating = $state(false);
	let deleting = $state([]);
</script>

...and toggle creating inside the first use:enhance:

src/routes/+page
<form
	method="POST"
	action="?/create"
	use:enhance={() => {
		creating = true;

		return async ({ update }) => {
			await update();
			creating = false;
		};
	}}
>
	<label>
		add a todo:
		<input
			disabled={creating}
			name="description"
			value={form?.description ?? ''}
			autocomplete="off"
			required
		/>
	</label>
</form>

We can then show a message while we’re saving data:

src/routes/+page
<ul class="todos">
	<!-- ... -->
</ul>

{#if creating}
	<span class="saving">saving...</span>
{/if}

In the case of deletions, we don’t really need to wait for the server to validate anything — we can just update the UI immediately:

src/routes/+page
<ul class="todos">
	{#each data.todos.filter((todo) => !deleting.includes(todo.id)) as todo (todo.id)}
		<li in:fly={{ y: 20 }} out:slide>
			<form
				method="POST"
				action="?/delete"
				use:enhance={() => {
					deleting = [...deleting, todo.id];
					return async ({ update }) => {
						await update();
						deleting = deleting.filter((id) => id !== todo.id);
					};
				}}
			>
				<input type="hidden" name="id" value={todo.id} />
				<button aria-label="Mark as complete">✔</button>

				{todo.description}
			</form>
		</li>
	{/each}
</ul>

use:enhance is very customizable — you can cancel() submissions, handle redirects, control whether the form is reset, and so on. See the docs for full details.

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<script>
	import { fly, slide } from 'svelte/transition';
	import { enhance } from '$app/forms';
 
	let { data, form } = $props();
</script>
 
<div class="centered">
	<h1>todos</h1>
 
	{#if form?.error}
		<p class="error">{form.error}</p>
	{/if}
 
	<form method="POST" action="?/create" use:enhance>
		<label>
			add a todo:
			<input
				name="description"
				value={form?.description ?? ''}
				autocomplete="off"
				required
			/>
		</label>
	</form>
 
	<ul class="todos">
		{#each data.todos as todo (todo.id)}
			<li in:fly={{ y: 20 }} out:slide>
				<form method="POST" action="?/delete" use:enhance>
					<input type="hidden" name="id" value={todo.id} />
					<span>{todo.description}</span>
					<button aria-label="Mark as complete"></button>
				</form>
			</li>
		{/each}
	</ul>
</div>
 
<style>
	.centered {
		max-width: 20em;
		margin: 0 auto;
	}
 
	label {
		width: 100%;
	}
 
	input {
		flex: 1;
	}
 
	span {
		flex: 1;
	}
 
	button {
		border: none;
		background: url(./remove.svg) no-repeat 50% 50%;
		background-size: 1rem 1rem;
		cursor: pointer;
		height: 100%;
		aspect-ratio: 1;
		opacity: 0.5;
		transition: opacity 0.2s;
	}
 
	button:hover {
		opacity: 1;
	}
 
	.saving {
		opacity: 0.5;
	}
</style>