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Conclusion
Basic SvelteKit API routes POST handlers

You can also add handlers that mutate data, such as POST. In most cases, you should use form actions instead — you’ll end up writing less code, and it’ll work without JavaScript, making it more resilient.

Inside the keydown event handler of the ‘add a todo’ <input>, let’s post some data to the server:

src/routes/+page
<input
	type="text"
	autocomplete="off"
	onkeydown={async (e) => {
		if (e.key !== 'Enter') return;

		const input = e.currentTarget;
		const description = input.value;

		const response = await fetch('/todo', {
			method: 'POST',
			body: JSON.stringify({ description }),
			headers: {
				'Content-Type': 'application/json'
			}
		});

		input.value = '';
	}}
/>

Here, we’re posting some JSON to the /todo API route — using a userid from the user’s cookies — and receiving the id of the newly created todo in response.

Create the /todo route by adding a src/routes/todo/+server.js file with a POST handler that calls createTodo in src/lib/server/database.js:

src/routes/todo/+server
import { json } from '@sveltejs/kit';
import * as database from '$lib/server/database.js';

export async function POST({ request, cookies }) {
	const { description } = await request.json();

	const userid = cookies.get('userid');
	const { id } = await database.createTodo({ userid, description });

	return json({ id }, { status: 201 });
}

As with load functions and form actions, the request is a standard Request object; await request.json() returns the data that we posted from the event handler.

We’re returning a response with a 201 Created status and the id of the newly generated todo in our database. Back in the event handler, we can use this to update the page:

src/routes/+page
<input
	type="text"
	autocomplete="off"
	onkeydown={async (e) => {
		if (e.key !== 'Enter') return;

		const input = e.currentTarget;
		const description = input.value;

		const response = await fetch('/todo', {
			method: 'POST',
			body: JSON.stringify({ description }),
			headers: {
				'Content-Type': 'application/json'
			}
		});

		const { id } = await response.json();

		data.todos = [...data.todos, {
			id,
			description
		}];

		input.value = '';
	}}
/>

You should only mutate data in such a way that you’d get the same result by reloading the page.

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<script>
	let { data } = $props();
</script>
 
<div class="centered">
	<h1>todos</h1>
 
	<label>
		add a todo:
		<input
			type="text"
			autocomplete="off"
			onkeydown={async (e) => {
				if (e.key !== 'Enter') return;
 
				const input = e.currentTarget;
				const description = input.value;
				
				// TODO handle submit
 
				input.value = '';
			}}
		/>
	</label>
 
	<ul class="todos">
		{#each data.todos as todo (todo.id)}
			<li>
				<label>
					<input
						type="checkbox"
						checked={todo.done}
						onchange={async (e) => {
							const done = e.currentTarget.checked;
 
							// TODO handle change
						}}
					/>
					<span>{todo.description}</span>
					<button
						aria-label="Mark as complete"
						onclick={async (e) => {
							// TODO handle delete
						}}
					></button>
				</label>
			</li>
		{/each}
	</ul>
</div>
 
<style>
	.centered {
		max-width: 20em;
		margin: 0 auto;
	}
 
	label {
		display: flex;
		width: 100%;
	}
 
	input[type="text"] {
		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;
	}
</style>