40-web-standards.md

title
Web standards

Throughout this documentation, you'll see references to the standard Web APIs that SvelteKit builds on top of. Rather than reinventing the wheel, we use the platform, which means your existing web development skills are applicable to SvelteKit. Conversely, time spent learning SvelteKit will help you be a better web developer elsewhere.

These APIs are available in all modern browsers and in many non-browser environments like Cloudflare Workers, Deno, and Vercel Functions. During development, and in adapters for Node-based environments (including AWS Lambda), they're made available via polyfills where necessary (for now, that is — Node is rapidly adding support for more web standards).

In particular, you'll get comfortable with the following:

Fetch APIs

SvelteKit uses fetch for getting data from the network. It's available in hooks and server routes as well as in the browser.

[!NOTE] A special version of fetch is available in load functions, server hooks and API routes for invoking endpoints directly during server-side rendering, without making an HTTP call, while preserving credentials. (To make credentialled fetches in server-side code outside load, you must explicitly pass cookie and/or authorization headers.) It also allows you to make relative requests, whereas server-side fetch normally requires a fully qualified URL.

Besides fetch itself, the Fetch API includes the following interfaces:

Request

An instance of Request is accessible in hooks and server routes as event.request. It contains useful methods like request.json() and request.formData() for getting data that was posted to an endpoint.

Response

An instance of Response is returned from await fetch(...) and handlers in +server.js files. Fundamentally, a SvelteKit app is a machine for turning a Request into a Response.

Headers

The Headers interface allows you to read incoming request.headers and set outgoing response.headers. For example, you can get the request.headers as shown below, and use the json convenience function to send modified response.headers:

// @errors: 2461
/// file: src/routes/what-is-my-user-agent/+server.js
import { json } from '@sveltejs/kit';

/** @type {import('./$types').RequestHandler} */
export function GET({ request }) {
	// log all headers
	console.log(...request.headers);

	// create a JSON Response using a header we received
	return json({
		// retrieve a specific header
		userAgent: request.headers.get('user-agent')
	}, {
		// set a header on the response
		headers: { 'x-custom-header': 'potato' }
	});
}

FormData

When dealing with HTML native form submissions you'll be working with FormData objects.

// @errors: 2461
/// file: src/routes/hello/+server.js
import { json } from '@sveltejs/kit';

/** @type {import('./$types').RequestHandler} */
export async function POST(event) {
	const body = await event.request.formData();

	// log all fields
	console.log([...body]);

	return json({
		// get a specific field's value
		name: body.get('name') ?? 'world'
	});
}

Stream APIs

Most of the time, your endpoints will return complete data, as in the userAgent example above. Sometimes, you may need to return a response that's too large to fit in memory in one go, or is delivered in chunks, and for this the platform provides streams — ReadableStream, WritableStream and TransformStream.

URL APIs

URLs are represented by the URL interface, which includes useful properties like origin and pathname (and, in the browser, hash). This interface shows up in various places — event.url in hooks and server routes, page.url in pages, from and to in beforeNavigate and afterNavigate and so on.

URLSearchParams

Wherever you encounter a URL, you can access query parameters via url.searchParams, which is an instance of URLSearchParams:

// @filename: ambient.d.ts
declare global {
	const url: URL;
}

export {};

// @filename: index.js
// ---cut---
const foo = url.searchParams.get('foo');

Web Crypto

The Web Crypto API is made available via the crypto global. It's used internally for Content Security Policy headers, but you can also use it for things like generating UUIDs:

const uuid = crypto.randomUUID();