What causes ""postMessage" requires at least one argument." in vitest-dev/vitest?

Pass an explicit message: `self.postMessage(payload)` — even `null` or `{}` is valid if you only need a signal.

vitest-dev/vitest · error · SyntaxError

"postMessage" requires at least one argument.

Error message

"postMessage" requires at least one argument.

What it means

The worker-side context object created by Vitest's Worker constructor (packages/web-worker/src/worker.ts:74-79) exposes a `postMessage` to code running INSIDE the worker (i.e. `self.postMessage(...)`). It mirrors the WHATWG DedicatedWorkerGlobalScope.postMessage contract, which requires a message argument. Calling it with zero arguments throws a SyntaxError, exactly like a real browser worker would.

Source

Thrown at packages/web-worker/src/worker.ts:76

        origin: typeof location !== 'undefined' ? location.origin : 'http://localhost:3000',
        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: (...args: any[]) => {
          return this._vw_workerTarget.removeEventListener(...args as [any, any])
        },
        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
        },
      }

      selfProxy = new Proxy(context, {

View on GitHub (pinned to d568f8ce37)

Solutions

Pass an explicit message: `self.postMessage(payload)` — even `null` or `{}` is valid if you only need a signal.
If the call is conditional, guard it so postMessage is only invoked when there is a payload to send.
Audit any wrapper around postMessage to ensure it forwards at least the first argument.

Example fix

// before — inside worker.js
function notify() {
  self.postMessage() // forgot the payload
}

// after
function notify(payload) {
  if (payload !== undefined) self.postMessage(payload)
}

Defensive patterns

Strategy: validation

Validate before calling

// Guard worker-side postMessage calls so they never run empty.
function postFromWorker(payload, transfer) {
  if (arguments.length === 0 && payload === undefined) {
    throw new TypeError('postFromWorker requires a payload')
  }
  const args = transfer ? [payload, transfer] : [payload ?? null]
  self.postMessage(...args)
}

Prevention

Always pass an explicit payload to self.postMessage — use `null` only when intentionally sending an empty signal.
Forward variadic args with a minimum-arity assertion in any wrapper.
Unit-test worker message helpers with the no-argument case to catch dropped payloads early.

When it happens

Trigger: Worker code calling `self.postMessage()` or `postMessage()` with no arguments; a conditional code path that calls postMessage only for its side effect; a wrapper/helper that forwards an arguments object and forwards zero args; off-by-one logic that drops the message payload before posting.

Common situations: Refactoring worker message handlers and accidentally dropping the payload; calling postMessage() inside a wrapper that destructures `(...args)` but receives none; mirroring a parent API that occasionally posts empty signals; testing edge cases where the worker has nothing to send yet still calls postMessage.

Related errors

AI-assisted analysis of vitest-dev/vitest@d568f8ce37 (2026-08-03). Data as JSON: /data/errors/0e27a57b5c4686a4.json. Report an issue: GitHub.