feat: node worker support out of the box

[prathameshnetake]

Support building of NodeJS worker out of the box. This PR also support dev mode
Successor of an old PR #3932

[aviallon]

I'd pay for this to be merged. Seriously.

[prathameshnetake]

@antfu @hi-ogawa please review

[GeoffreyBooth]

I’m eager to see this PR (or another that adds worker support) land. It feels like a glaring gap in Vite that it doesn’t support multithreaded Node apps.

In case it helps anyone else, in my SvelteKit app I use the following custom plugin to enable worker support; edit as needed for your own worker files and paths:

import { copyFile, mkdir, cp } from 'node:fs/promises'
import { resolve } from 'node:path'
import { env } from 'node:process'


/**
 * Vite plugin to copy worker files to the build directory after adapter-node finishes.
 * This ensures worker threads are available when running in preview or production mode.
 *
 * vite-plugin-node-worker outputs to .svelte-kit/output/server/, but adapter-node
 * doesn't automatically copy worker files to build/server/. This plugin handles that.
 * We also need to copy the chunks directory since the worker imports from it.
 */
export function copyWorkerToBuildPlugin() {
	return {
		name: 'copy-worker-to-build',
		/**
		 * Run after the adapter has finished writing files
		 */
		async closeBundle() {
			const outputDir = resolve(import.meta.dirname, '../../.svelte-kit/output/server')
			const sourceWorker = resolve(outputDir, 'my-worker.worker.mjs')
			const sourceChunks = resolve(outputDir, 'chunks')
			const targetDir = resolve(import.meta.dirname, '../../build/server')
			const targetWorker = resolve(targetDir, 'my-worker.worker.mjs')
			const targetChunks = resolve(targetDir, 'chunks')

			try {
				// Ensure target directory exists
				await mkdir(targetDir, { recursive: true })

				// Copy the worker file
				await copyFile(sourceWorker, targetWorker)

				// Copy the chunks directory (worker dependencies)
				// Use force: false to not overwrite existing chunks from adapter-node
				await cp(sourceChunks, targetChunks, { recursive: true, force: false })
			} catch (error) {
				// Only error if we're in a build context where the worker file should exist
				if ((error instanceof Error && 'code' in error && error.code !== 'ENOENT') || env.BUILDING) {
					console.error('Failed to copy worker file:', error)
				}
			}
		},
	}
}

[aviallon]

@antfu Is there something that should be done before it is mergeable?