At its core, SvelteKit’s job boils down to three things:

1. Routing — figure out which route matches an incoming request
2. Loading — get the data needed by the route
3. Rendering — generate some HTML (on the server) or update the DOM (in the browser)

We’ve seen how routing and rendering work. Let’s talk about the middle part — loading.

Every page of your app can declare a load function in a +page.server.js file alongside the +page.svelte file. As the file name suggests, this module only ever runs on the server, including for client-side navigations. Let’s add a src/routes/blog/+page.server.js file so that we can replace the hard-coded links in src/routes/blog/+page.svelte with actual blog post data:

import { posts } from './data.js';

export function load() {
	return {
		summaries: posts.map((post) => ({
			slug: post.slug,
			title: post.title
		}))
	};
}

For the sake of the tutorial, we’re importing data from src/routes/blog/data.js. In a real app, you’d be more likely to load the data from a database or a CMS, but for now we’ll do it like this.

We can access this data in src/routes/blog/+page.svelte via the data prop:

<script>
	let { data } = $props();
</script>

<h1>blog</h1>

<ul>
	<li><a href="/blog/one">one</a></li>
	<li><a href="/blog/two">two</a></li>
	<li><a href="/blog/three">three</a></li>
	{#each data.summaries as { slug, title }}
		<li><a href="/blog/{slug}">{title}</a></li>
	{/each}
</ul>

Now, let’s do the same for the post page:

import { posts } from '../data.js';

export function load({ params }) {
	const post = posts.find((post) => post.slug === params.slug);

	return {
		post
	};
}

<script>
	let { data } = $props();
</script>

<h1>blog post</h1>
<h1>{data.post.title}</h1>
<div>{@html data.post.content}</div>

There’s one last detail we need to take care of — the user might visit an invalid pathname like /blog/nope, in which case we’d like to respond with a 404 page:

import { error } from '@sveltejs/kit';
import { posts } from '../data.js';

export function load({ params }) {
	const post = posts.find((post) => post.slug === params.slug);

	if (!post) error(404);

	return {
		post
	};
}

We’ll learn more about error handling in later chapters.