vite-plugin-sveltekit-node-worker

Vite plugin that bundles Node.js worker_threads for SvelteKit 3 with adapter-node.

Import a worker with ?nodeWorker. The plugin bundles it with esbuild and returns a Worker subclass you can new.

It works in both development and production.

Installation

SvelteKit 3 is currently published as next (beta). This plugin needs that and adapter-node:

npm install @sveltejs/kit@next @sveltejs/adapter-node@next
npm install -D vite-plugin-sveltekit-node-worker

Usage

Add the plugin next to sveltekit():

import { sveltekit } from "@sveltejs/kit/vite";
import { defineConfig } from "vite";
import { sveltekitNodeWorker } from "vite-plugin-sveltekit-node-worker";

export default defineConfig({
  plugins: [sveltekit(), sveltekitNodeWorker()],
});

Match adapter-node's out if you change it:

sveltekitNodeWorker({ out: "dist" });

Write a worker, then import it with ?nodeWorker:

// src/lib/server/heavy.ts
import { parentPort } from "node:worker_threads";

parentPort?.on("message", (n: number) => {
  parentPort?.postMessage(n * 2);
});

// src/lib/server/spawner.ts
import Worker from "./heavy.ts?nodeWorker";

const worker = new Worker();
worker.postMessage(21);
worker.on("message", (result) => {
  console.log(result); // 42
});

The default export is a class that extends node:worker_threads.Worker and always spawns with { type: "module" }. Pass extra Worker options to the constructor if needed.

Client builds get export default null — Node workers only run on the server.

Worker modules

Worker source can import:

• #lib/... — resolved to src/lib (SvelteKit 3 subpath imports)
• $app/env/private and $app/env/public — shims from src/env.ts / src/env.js (defineEnvVars, including public and static)

.env files load in Vite order: .env.[mode].local, .env.[mode], .env.local, .env.

Dev vs production

Dev (vite dev): workers build to .svelte-kit/node-workers and spawn from that path. Changing a worker file or src/env rebuilds and full-reloads.

Production (vite build + adapter-node): the worker file is written next to the SSR chunk that imported it. After the adapter re-bundles, the plugin copies it beside any server chunk that references it under out (default build).

Source Code

Since this plugin is MIT licensed, you can also contribute to it at it's repo on GitHub