Fastify Svelte View
fastify-svelte-view
A Fastify plugin for rendering Svelte components with support for SSR (Server-Side Rendering), CSR (Client-Side Rendering), and SSR with hydration. This plugin makes it easy to integrate Svelte into your Fastify application, providing flexible rendering options for different use cases.
Note: Work in Progress
This is a quite new open-source project and still a work in progress. Ideas, feedback, and especially pull requests are very welcome!
Installation
npm install @matths/fastify-svelte-view
Registering the Plugin
You can register the plugin similar to all the other fastify plugins. Right now you should share the templateDir where your .svelte Files are as well as your layoutTemplate.
const path = require('path');
const fastify = require('fastify')();
const svelteView = require('fastify-svelte-view');
await fastify.register(svelteView, {
templateDir: path.join(__dirname, '../views'),
layoutTemplate: 'layout.html'
});
Layout template
You can specify a layout template (e.g., layout.html) to wrap your rendered Svelte components. The layout must include placeholders for the rendered HTML and scripts.
If you do not provide a layoutTemplate the plugin gracefully falls back to a default template.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<title>{% title %}</title>
{% head %}
</head>
<body>
<div id="app">{% body %}</div>
{% scripts %}
</body>
</html>
API
fastify.renderSvelte(options) renders a Svelte component with the given options.
Options
file: Path to a Svelte component file (relative totemplateDir).source: Inline Svelte component source code (string).props: Props to pass to the component.mode:'SSR'(default) |'CSR'.hydrate: Boolean, enables hydration (default) for SSR output.title: Optional, sets the page title.
You can use either file or source to specify the Svelte component.
Usage Examples
Rendering a Svelte Component (SSR + Hydration)
The Svelte component is rendered at the server on request. You can use dynamic content within your props. The complete HTML is rendered by the browser and hydrated through client side Javascript to enable user interaction.
fastify.get('/render', async (request, reply) => {
const html = await fastify.renderSvelte({
title: 'Svelte Page SSR',
props: { foo: 'bar' },
file: 'Example.svelte',
mode: 'SSR',
hydrate: true
});
reply.type('text/html').send(html);
});
Rendering a Svelte Component (CSR)
This time the Svelte code is provides as a string. Rendering mode is CSR, which means an almost empty HTML container is brought to life on the client side by JavaScript.
fastify.get('/render', async (request, reply) => {
const html = await fastify.renderSvelte({
title: 'Svelte Page CSR', // page title
props: { a: 1, b: 2 }, // props
mode: 'CSR',
source: `
<script>
import { onMount } from 'svelte';
let { a = 0, b = 0 } = $props();
let c = $derived(a + b);
let count = $state(0);
const getCount = async () => {
const result = await fetch('/count');
count = parseInt(await result.text());
};
onMount(getCount);
</script>
<h1>Demo</h1>
<span>a + b = {c}</span>
<p>This page has been viewed {count} times.</p>
`
});
reply.type('text/html').send(html);
});
Modes
Here's an overview of the architectural differences between the modes to sharpen your mental model.
SSR (Server-Side Rendering):
- The server renders the Svelte component to HTML and sends it to the client.
- No interactivity unless hydrated; great for SEO and fast initial paint.
SSR + Hydration:
- The server renders HTML, then the client loads JavaScript to "hydrate" the page, making it interactive.
- Best of both worlds: fast initial load, SEO, and interactivity.
CSR (Client-Side Rendering):
- The server sends a minimal HTML shell and JavaScript bundle to the client.
- Svelte runs entirely in the browser, rendering the UI after page load.
- Less SEO, usually for highly interactive apps or SPA (single page applications).
SSG (Server-Side Generation): - not included
- SSG would happen at build time (when the fastify server is started). Feel free to share your ideas about stuff like this with us.
- fastify-svelte-view currently does not provide any caching. We might add this later on. There are ideas for caching, cache-warmup and cache invalidation, but no implementation, yet.
Acknowledgements & Dependencies
This project is built on top of several important open-source packages, e.g.:
- fastify: The core web framework providing the server and routing foundation.
- svelte: The modern UI framework enabling both SSR and CSR rendering of components.
- rollup: The module bundler used for compiling and bundling Svelte components and assets.
- @piscina/fastify: For efficient worker thread management and performance.
Thanks to the great open source maintainers and communities, for their contributions and inspiration that make projects like this possible.
License
MIT
