Building your app

Building a SvelteKit app happens in two stages, which both happen when you run vite build (usually via npm run build).

Firstly, Vite creates an optimized production build of your server code, your browser code, and your service worker (if you have one). Prerendering is executed at this stage, if appropriate.

Secondly, an adapter takes this production build and tunes it for your target environment — more on this on the following pages.

During the build

SvelteKit will load your +page/layout(.server).js files (and all files they import) for analysis during the build. Any code that should not be executed at this stage must check that building from $app/environment is false:

import { const building: boolean
SvelteKit analyses your app during the build step by running it. During this process, building is true. This also applies during prerendering.
building } from '$app/environment';
import { import setupMyDatabasesetupMyDatabase } from '$lib/server/database';

if (!const building: boolean
SvelteKit analyses your app during the build step by running it. During this process, building is true. This also applies during prerendering.
building) {
	import setupMyDatabasesetupMyDatabase();
}

export function function load(): voidload() {
	// ...
}

Preview your app

After building, you can view your production build locally with vite preview (via npm run preview). Note that this will run the app in Node, and so is not a perfect reproduction of your deployed app — adapter-specific adjustments like the platform object do not apply to previews.