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Basic Svelte
Introduction
Reactivity
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Advanced Svelte
Advanced reactivity
Reusing content
Motion
Advanced bindings
Advanced transitions
Context API
Special elements
<script module>
Next steps
Basic SvelteKit
Introduction
Routing
Loading data
Headers and cookies
Shared modules
Forms
API routes
$app/state
Errors and redirects
Advanced SvelteKit
Hooks
Page options
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Advanced routing
Advanced loading
Environment variables
Conclusion
Basic SvelteKit Forms Progressive enhancement

Because we’re using <form>, our app works even if the user doesn’t have JavaScript (which happens more often than you probably think). That’s great, because it means our app is resilient.

Most of the time, users do have JavaScript. In those cases, we can progressively enhance the experience, the same way SvelteKit progressively enhances <a> elements by using client-side routing.

Import the enhance function from $app/forms...

src/routes/+page
<script>
	import { enhance } from '$app/forms';

	let { data, form } = $props();
</script>
<script lang="ts">
	import { enhance } from '$app/forms';

	let { data, form } = $props();
</script>

...and add the use:enhance directive to the <form> elements:

src/routes/+page
<form method="POST" action="?/create" use:enhance>
src/routes/+page
<form method="POST" action="?/delete" use:enhance>

And that’s all it takes! Now, when JavaScript is enabled, use:enhance will emulate the browser-native behaviour except for the full-page reloads. It will:

  • update the form prop
  • invalidate all data on a successful response, causing load functions to re-run
  • navigate to the new page on a redirect response
  • render the nearest error page if an error occurs

Now that we’re updating the page rather than reloading it, we can get fancy with things like transitions:

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

	let { data, form } = $props();
</script>
<script lang="ts">
	import { fly, slide } from 'svelte/transition';
	import { enhance } from '$app/forms';

	let { data, form } = $props();
</script>
src/routes/+page
<li in:fly={{ y: 20 }} out:slide>...</li>

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<script>
	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">
		<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>
				<form method="POST" action="?/delete">
					<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>