Upload file in Sveltekit

sveltekit is trending nowadays and I believe it totally deserves it. I tried it for one of my side projects and enjoyed it.

sveltekit is a fullstack meta-framework for sveltejs that can handle almost everything we need for developing a web application. one of these features is file uploading and storing it to file system.
for uploading images (or other types of files) in sveltekit, we have 2 options. form actions, and API routes.

template

in +page.svelte we can create a form with multipart/form-data as its enctype. this form submits to an action handler (that we are to implement in a moment):

<form action="?/avatar" method="post" enctype="multipart/form-data">
  <input type="file" name="avatar" placeholder="avatar" />
  <button type="submit">upload</button>
</form>

Action Handler

as the sveltekit documentation says:

actions allow you to POST data to the server using the <form> element.

basically, they are functions that run on the server and are specifically designed for accepting form actions. you can read more about them in sveltekit documentation

we have to create an action handler in +page.server.ts. this file can be used for providing page initial data (by exporting a function named load) and defining form action handlers.
note that as specified in <form /> tag, the action name should be avatar.

import fs from 'fs/promises'
import path from 'path'
import { redirect } from '@sveltejs/kit'
import type { PageLoad, Actions } from './$types'

export const actions: Actions = {
  avatar: async ({ request, route, url }) => {
    try {
      const data = Object.fromEntries(await request.formData())
      const filePath = path.join(
        process.cwd(),
        'static',
        'avatars',
        `${crypto.randomUUID()}.${(data.avatar as Blob).type.split('/')[1]}`
      )
      await fs.writeFile(filePath, Buffer.from(await (data.avatar as Blob).arrayBuffer()))

      // TODO: store the file path in database for further references.
      throw redirect(303, url.pathname)
    } catch (err) {
      throw fail(500, { err: err })
    }
  },
}

that's it! now the uploaded file will be stored in the static folder and it's accessible like other static assets.

also, I used crypto module for naming files in the file system instead of the original file name, for having the same naming format for all files and removing long file names.

note that this code is not completed logic. the path of the uploaded file should be stored in a persisted storage or database for further usage. also, errors should be handled properly.

---

API routes

the same functionality can be applied as an API route as well. forms should be submitted using javascript in this way. also if you have multiple clients besides your client-side sveltekit app (like a react-native mobile app) this approach is the one should choose.

we can define API route endpoints in +server.ts. for specifying handlers for different HTTP methods (GET, POST, etc.) we can declare a function with the name of the HTTP verb. here we want a handler for

import path from 'path'
import fs from 'fs/promises'
import { fail, redirect } from '@sveltejs/kit';
import type { RequestHandler } from './$types';

export const POST = (async ({ request, url }) => {
    try {
        const data = Object.fromEntries(await request.formData())
        const filePath = path.join(
            process.cwd(),
            "static",
            "avatars",
            `${crypto.randomUUID()}.${((data.avatar as Blob).type.split("/")[1])}`
        );
        await fs.writeFile(filePath, Buffer.from(await (data.avatar as Blob).arrayBuffer()))
            // TODO: store the file path in database for further references.
        return new Response(String({path: filePath}))
    } catch (err) {
        throw fail(500, { err: err })
    }
}) satisfies RequestHandler;

Note that in API routes the response body should be generated with Response object (which is global to typescript) so it's a little different from form actions.

you can read more about the API routes in sveltekit routing documentation.

---

after all, if you need to have static file serving, you should change vite.config.js in the root of the project a little:

import { sveltekit } from '@sveltejs/kit/vite';
import { searchForWorkspaceRoot } from 'vite'

/** @type {import('vite').UserConfig} */
const config = {
    plugins: [sveltekit()],
    server: {
        fs: {
            allow: [searchForWorkspaceRoot(process.cwd()), "/static/avatars/"]
        }
    },
    test: {
        include: ['src/**/*.{test,spec}.{js,ts}']
    }
};

export default config;

---

Although this method works fine for small apps, storing assets and serving them through static path is not considered a best practice. it's not about sveltekit limitations, using an object storage service like Amazon S3 or MinIO is preferred for larger applications.

hope you find this post useful!