API routes / POST handlers • Svelte Tutorial

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();

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

		data = { ...data, todos };

		input.value = '';
	}}
/>

You should only update data in such a way that you’d get the same result by reloading the page. The data prop is not deeply reactive, so you need to replace it — mutations like data.todos = todos will not cause a re-render.

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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>

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