SvelteKit is a framework for rapidly developing robust, performant web applications using Svelte. If you're familiar with React, you can think of SvelteKit as similar to Next. If you're coming from Vue, SvelteKit is similar to Nuxt. SvelteKit provides basic functionality like a router and server-side rendering (SSR), but it also handles build optimizations, offline support, preloading pages, and configurable rendering. It leverages Vite with a Svelte plugin to do Hot Module Replacement (HMR), reflecting changes to your code in the browser instantly for a lightning-fast and feature-rich development experience.

In this blog post, we will focus on a key feature of SvelteKit: load functions. We'll explore both client-side and server-side load functions, their differences, and when to use each.

What are Load Functions?

Load functions in SvelteKit are used to load data for a page. They run on the server before the page is rendered and can run on the client when navigating to the page after the first load. The data returned by the load function is available to the page component as a load prop.

The load function is an async function that receives an object with several properties:

• fetch: a function similar to window.fetch but works on both the server and the client
• page: an object containing information about the current page
• context: an object that you can populate with data in handle hooks and read in load functions
• session: the session object, if any

The load function can return an object with several properties:

• status: the HTTP status code. If not provided, it defaults to 200
• error: an error message
• redirect: a URL to redirect to
• props: an object that is passed as props to the page component
• context: an object that is available to load functions downstream

Client-Side Load Functions

Client-side load functions are defined in a page.ts file and are run in the browser. They are called every time a user navigates to the route after the initial page load. Here's an example:

import type { PageLoad } from './$types'

/**
 * @param {PageLoad} pageLoad
 * @returns {PageLoad}
 */
export async function load({ page, fetch, session }: PageLoad): Promise {
  // Fetch data here
  const response = await fetch('/api/data')
  const data = await response.json()

  return {
    props: { data }
  }
}

In this example, the load function fetches data from an API and returns it as a prop to the page component.

Server-Side Load Functions

Server-side load functions are defined in a page.server.ts file and are run on the server during the initial page load. They are not called on client-side navigation. Here's an example:

import type { PageServerLoad } from './$types'

/**
* @param {PageServerLoad} pageServerLoad
* @returns {PageServerLoad}
*/
export async function load({ page, fetch, session }: PageServerLoad): Promise {
    // Fetch data here
    const response = await fetch('/api/data')
    const data = await response.json()

    return {
        props: { data }
    };
}

In this example, the load function fetches data from an API and returns it as a prop to the page component.

Choosing Between Client-Side and Server-Side

Choosing between client-side and server-side load functions depends on the specific needs of your application. If you need to fetch data that changes often and needs to be up-to-date every time a user navigates to the page, a client-side load function would be appropriate. If the data is static and doesn't change often, a server-side load function would be more appropriate as it can take advantage of server-side rendering for faster initial page loads.

Conclusion

SvelteKit's load functions provide a powerful and flexible way to load data for your pages. Whether you choose to use client-side or server-side load functions will depend on the specific needs of your application. By understanding the differences between these two types of load functions and how they work, you can make more informed decisions about how to structure your SvelteKit application.