Incorrect coverage in web workers with @vitest/web-worker and @vitest/coverage-v8

[robojumper]

Describe the bug

I'm using @vitest/web-worker to test a web worker. @vitest/coverage-v8 reports incorrect coverage metrics, while @vitest/coverage-istanbul seems to work fine.

Reproduction

https://github.com/robojumper/vitest-web-worker-v8-coverage-bug - run npm run test:coverage and view the coverage report for src/worker/worker.ts:

v8:

istanbul:

istanbul coverage looks correct while v8 coverage is completely wrong.

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (12) x64 AMD Ryzen 5 3600 6-Core Processor
    Memory: 18.36 GB / 31.91 GB
  Binaries:
    Node: 22.13.1 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.19 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 10.9.0 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.6.0 - ~\AppData\Roaming\npm\pnpm.CMD
  Browsers:
    Edge: Chromium (132.0.2957.127)
    Internet Explorer: 11.0.19041.4355
  npmPackages:
    @vitest/coverage-istanbul: ^3.0.4 => 3.0.4
    @vitest/coverage-v8: ^3.0.4 => 3.0.4
    @vitest/ui: ^3.0.4 => 3.0.4
    @vitest/web-worker: ^3.0.4 => 3.0.4
    vite: ^6.0.0 => 6.0.11
    vitest: ^3.0.4 => 3.0.4

Used Package Manager

npm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[AriPerkkio]

The hard-coded execution wrapper doesn't match @vitest/web-worker's context lenght:

vitest/packages/coverage-v8/src/provider.ts

Lines 31 to 32 in 1154662

	// TODO: vite-node should export this
	const WRAPPER_LENGTH = 185

Custom context is created at:

vitest/packages/web-worker/src/runner.ts

Lines 11 to 17 in 1154662

	prepareContext(context: Record<string, any>) {
	const ctx = super.prepareContext(context)
	// not supported for now, we can't synchronously load modules
	return Object.assign(ctx, this.context, {
	importScripts,
	})
	}

vitest/packages/web-worker/src/worker.ts

Lines 50 to 97 in 1154662

	const context = {
	onmessage: null as null | ((e: MessageEvent) => void),
	onmessageerror: null,
	onerror: null,
	onlanguagechange: null,
	onoffline: null,
	ononline: null,
	onrejectionhandled: null,
	onrtctransform: null,
	onunhandledrejection: null,
	origin: typeof location !== 'undefined' ? location.origin : 'http://localhost:3000',
	importScripts: () => {
	throw new Error(
	'[vitest] `importScripts` is not supported in Vite workers. Please, consider using `import` instead.',
	)
	},
	crossOriginIsolated: false,
	name: options?.name || '',
	close: () => this.terminate(),
	dispatchEvent: (event: Event) => {
	return this._vw_workerTarget.dispatchEvent(event)
	},
	addEventListener: (...args: any[]) => {
	if (args[1]) {
	this._vw_insideListeners.set(args[0], args[1])
	}
	return this._vw_workerTarget.addEventListener(...args as [any, any])
	},
	removeEventListener: this._vw_workerTarget.removeEventListener,
	postMessage: (...args: any[]) => {
	if (!args.length) {
	throw new SyntaxError(
	'"postMessage" requires at least one argument.',
	)
	}
	debug(
	'posting message %o from the worker %s to the main thread',
	args[0],
	this._vw_name,
	)
	const event = createMessageEvent(args[0], args[1], cloneType())
	this.dispatchEvent(event)
	},
	get self() {
	return selfProxy
	},
	}

We should probably pass the WRAPPER_LENGTH from test runner threads to main thread when reporting coverage results. 🤔

Changing the WRAPPER_LENGTH to 429 fixes the issue: