Using WebSockets With SvelteKit

Published May 1, 2022

Table of Contents

Using WebSockets

By the end of this post you’re going to learn how to make WebSockets work in SvelteKit for development and production.

If you’re reading this in the future SvelteKit might have native support for WebSockets similar to how it does HTTP request methods so it might be worth checking the documentation.

export const  GET: RequestHandler = async () => { /* ... */ }

export const  POST: RequestHandler = async () => { /* ... */ }

// This doesn't exist yet unfortunately 😅
export const WS: WebSocketRequestHandler = async () => { /* ... */ }

You can read more about it inside the discussion about native support for WebSockets inside SvelteKit if you want.

One solution is to create a separate server but then you have to run two things on separate ports so let me show you how you can do it using SvelteKit instead.

WebSockets For Development

To make use of WebSockets during development you need to write a simple Vite plugin that hooks into the Vite development server.

Thanks to Bob Fanger for his workaround.

This might sound intimidating but it’s not — trust me I’m not an expert. 😄

Vite has a configureServer hook which lets you configure the development server.

This is the code for the Vite plugin.

const plugin = {
  name: 'plugin',
  configureServer(server) {
    // we can pass the server to
    const io = new Server(server.httpServer)
    // ...

That’s it!

There’s more code in the final version but you can use any WebSocket library you prefer.

To avoid backtracking install the Node adapter with npm i -D @sveltejs/adapter-node because it’s required for this to work.

Add the plugin code inside svelte.config.js or extract it into a separate file.

import adapter from '@sveltejs/adapter-node'
import preprocess from 'svelte-preprocess'

import { Server } from ''

const webSocketServer = {
  name: 'webSocketServer',
  configureServer(server) {
    const io = new Server(server.httpServer)

    io.on('connection', (socket) => {
      socket.emit('eventFromServer', 'Hello, World 👋')

/** @type {import('@sveltejs/kit').Config} */
const config = {
  preprocess: preprocess(),
  kit: {
    adapter: adapter(),
    vite: {
      plugins: [webSocketServer],

export default config

🐿️ Use a dynamic import if you need to import code that’s not a module inside webSocketServer for example const code = await import('./code.js') otherwise you’re going to encounter an error.

Here’s an example how this works using on the client.

<script lang="ts">
  import { io } from ''

  const socket = io()

  socket.on('eventFromServer', (message) => {

If you start the development server with npm run dev it should work and you should see the Hello, World 👋 message in the console!

If you want to emmit a message when a value changes don’t forget you can use a reactive block.

<script lang="ts">
  // ...

  $: {
    // send message to server
    socket.emit('eventFromClient', $reactiveValue)

WebSockets For Production

Thanks to the Sveltekit Node adapter you can create a custom server and do whatever you want.

I’m going to use Express because it’s a minimal web framework.

npm i express @types/express

The Node adapter creates the index.js and handler.js files in your build folder when you run npm run build — creating a custom server works by importing the handler from build/handler.js and using your custom server instead of index.js.

Make sure you create the build folder.

npm run build

I’ve created the server/index.js file at the root of the project for convenience but this depends on your build step and where you’re deploying it.

import express from 'express'
import { createServer } from 'http'
import { Server } from ''

import { handler } from '../build/handler.js'

const port = 3000
const app = express()
const server = createServer(app)

const io = new Server(server)

io.on('connection', (socket) => {
  socket.emit('eventFromServer', 'Hello, World 👋')

// SvelteKit should handle everything else using Express middleware


Add the script to start the server inside package.json.

  "scripts": {
    "start": "node ./server"
    // ...

That’s it! 🥳

You can run npm start and everything should work.

This looks similar to the Vite plugin from earlier but it’s just a simple Express server.

If you need to pass environment variables I have a post on using environment variables in SvelteKit.

Thanks for reading! 🏄️


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