TypeScript

All APIs in SvelteKit are fully typed. Additionally, it's possible to tell SvelteKit how to type objects inside your app by declaring the App namespace. By default, a new project will have a file called src/app.d.ts containing the following:

/// <reference types="@sveltejs/kit" />

declare namespace App {
	interface Locals {}

	interface Platform {}

	interface Session {}

	interface Stuff {}
}

By populating these interfaces, you will gain type safety when using event.locals, event.platform, session and stuff:

App.Locals

The interface that defines event.locals, which can be accessed in hooks (handle, handleError and getSession) and endpoints.

App.Platform

If your adapter provides platform-specific context via event.platform, you can specify it here.

App.Session

The interface that defines session, both as an argument to load functions and the value of the session store.

App.Stuff

The interface that defines stuff, as input or output to load or as the value of the stuff property of the page store.