Creating a Prerendered Svelte Web Application with Client-side Data Loading

A comprehensive guide for static rendering and dynamic client-side updates

Key Insights

• Static Site Generation: Use static adapters like adapter-static to generate pre-rendered HTML at build time.
• Client-side Data Loading: Leverage Svelte’s onMount lifecycle and load functions to fetch dynamic content in the browser.
• SSR-free Environment: Ensure that server-side rendering does not run post-build by avoiding server-only code in critical components.

Understanding Static Site Generation in Svelte

When you create a Svelte web application intended to operate as a fully static site, you opt for prerendering your pages. This means that the HTML files for your application's pages are generated at build time. The key benefit of this approach is an ultra-fast initial load, improved search engine optimization (SEO), and reduced dependency on a server environment.

Adapter Configuration for Static Generation

For a Svelte application that is to be served as a static website, using adapter-static is the standard approach. By configuring the adapter in your svelte.config.js file, you essentially convert your project into one that outputs pure static HTML, CSS, and JavaScript files.

Example: Configuring adapter-static

In your svelte.config.js file, include the following configuration:

// Import the adapter
import adapter from '@sveltejs/adapter-static';

export default {
  kit: {
    // Configure adapter-static for generating static sites
    adapter: adapter({
      pages: 'build',       // Output folder for HTML pages
      assets: 'build',      // Output folder for assets like CSS and images
      fallback: undefined,  // Optional: specify a single-page app fallback if needed
      precompress: false,   // Optional: if you wish to precompress files
      strict: true
    })
  }
};
  

With this configuration, when you run the Svelte build command, you'll generate a collection of static files that represent your application's pages.

Prerendering Pages without Server-Side Components

Once the static adapter is configured, you can opt to pre-render your pages entirely. By setting the prerender option in your page components, you ensure that the HTML output is generated at build time. This means that there is no server-side component actively rendering these pages when a user accesses them.

Configuration in Page Files

To instruct SvelteKit to prerender your components, simply include the following declaration in your page’s script:

Example: Prerender Option

// In +page.js or +layout.js
export const prerender = true;
  

This single line ensures that during the build process, SvelteKit captures a static snapshot of your page.

In cases where additional control is needed – such as managing dynamic routes or excluding specific pages from prerendering – you may set the option to "auto" or manage it on a per-route basis. However, by setting it explicitly to true, you're effectively disabling any ongoing server-side rendering or runtime HTML generation.

Client-side Data Loading after Initial Prerendering

Even though your pages are served as static content, many web applications require dynamic updates – whether it’s fetching new content from an API or improving interactivity based on real-time data. Since you have already generated the basic structure and content at build time, dynamic data can be loaded when the page is rendered in the browser.

Using the onMount Lifecycle Hook

Svelte provides the onMount lifecycle hook which executes only on the client-side after a component has been mounted to the Document Object Model (DOM). By utilizing this hook, you can fetch updated or dynamic data once the static page is already loaded in the browser.

Example: Loading Data with onMount

// Example in a +page.svelte file
<script>
  import { onMount } from 'svelte';

  let dynamicData = [];
  let loading = true;

  async function fetchData() {
    try {
      const res = await fetch('https://api.example.com/data');
      dynamicData = await res.json();
    } catch (error) {
      console.error('Error fetching data:', error);
    } finally {
      loading = false;
    }
  }

  onMount(fetchData);
</script>

<template>
  <h1>Dynamic Content Loaded in Browser</h1>
  {#if loading}
    <p>Loading data...</p>
  {:else}
    <ul>
      {#each dynamicData as item}
        <li>{item.name}</li>
      {/each}
    </ul>
  {/if}
</template>
  

In this example, the data retrieval process occurs entirely on the client-side once the page has been loaded. This ensures that your initial page load is fast (thanks to prerendering) and user interactivity is maintained with up-to-date content.

Using Load Functions for Client-side Data Fetching

Alternatively, if there's a need to use SvelteKit's load function, you can define a client-centric data loader in your +page.js file. While the load function can run on both the server and client, in a static site scenario, be cautious to ensure that truly dynamic data is fetched on the client side.

Load Function Example

// In +page.js
export const prerender = true;  // Generate static HTML at build time

export function load() {
  return fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(data => {
      return { props: { data } };
    })
    .catch(error => {
      console.error('Error fetching data in load function:', error);
      return { props: { data: [] } };
    });
}
  

Note that if you intend for the data to update on the client side after the initial rendering, employing the onMount approach is typically preferred. The load function in a static adapter scenario is useful for building the initial render, but for subsequent updates without reloading the page, client-side lifecycle hooks are ideal.

Handling Dynamic Routes with Pre-rendering

For applications that include dynamic routes (for example, blog posts with dynamic slugs), pre-rendering requires a little extra attention. SvelteKit must be aware of all the possible routes during the build process. If the framework is not explicitly informed of every dynamic route, some pages might fail to generate correctly.

Ensuring Dynamic Routes are Recognized

You have two primary options to handle dynamic routes during prerendering:

• Manual Linking: Include links to all dynamic pages within your site’s navigation or content. This helps SvelteKit discover them automatically during the build process.
• Configuring Pre-render Entries: Manually list dynamic routes in your svelte.config.js under kit.prerender.entries to explicitly instruct SvelteKit to render those pages.

Example: Adding Dynamic Routes

// In svelte.config.js
export default {
  kit: {
    adapter: adapter({...}),
    prerender: {
      entries: [
        '/',
        '/about',
        '/blog',
        '/blog/post-1',
        '/blog/post-2'
        // add other dynamic routes as necessary
      ]
    }
  }
};
  

This configuration ensures that all specified routes are prerendered, avoiding missing pages at runtime.

A Comprehensive Overview: Combining Pre-rendering and Dynamic Data Loading

The overall strategy for creating a Svelte web application that leverages static site generation while still allowing dynamic updates in the browser revolves around the following key steps:

Step-by-Step Summary Table

import adapter from '@sveltejs/adapter-static'; 
export default { kit: { adapter: adapter({...}) } };

export const prerender = true;

import { onMount } from 'svelte'; 
onMount(() => { fetch(...); });

prerender: { entries: ['/','/about','/blog',...] }

export function load() { return fetch(...).then(...); }

Additional Tips and Best Practices

Avoid Server-side Code in Client Data Loading

When your objective is to maintain a fully static website without any active server-side rendering at runtime, it is crucial not to incorporate any server-specific operations in your components. This means avoiding the use of +page.server.js files for operations that are intended to run on the client. Use client-centric code blocks and client-only library functions to manage dynamic data.

Caching and Optimization

Although your pages are pre-rendered, if you fetch data from external APIs, consider caching responses to minimize latency and load times during client-side fetch operations. Browser caching, service workers, and other client-side optimization techniques can significantly enhance user experience.

Also, if you’re fetching data that does not change frequently, you might consider integrating that data during the build process with more advanced static generation strategies rather than refetching it every page load.

Testing Your Static Website

Be sure to test locally by building your SvelteKit application and serving the static output folder using a simple static server. This practice will help you catch any edge cases related to dynamic routing or client-side data issues before deploying to a production environment.

Example command for local testing:

// For example, using serve globally installed via npm
npx serve build
  

References

• Static Adapter - Svelte Documentation
• Page Options - SvelteKit Docs
• SvelteKit SSG: How to Prerender a SvelteKit Site - Rodney Lab
• A Deep Dive into SvelteKit’s Rendering Techniques - This Dot Co
• How to Create a SPA using SvelteKit Without Node or Server Side Routing - Matthew Moisen

Recommended Related Queries

• How to configure adapter-static in SvelteKit
• Client-side data fetching using onMount in Svelte
• Handling dynamic routes in SvelteKit with prerendering
• Differences between SSR and CSR in Svelte
• Best practices for static site optimization in SvelteKit