grafana/k6 · error

last argument to ws.connect must be a function

Error message

last argument to ws.connect must be a function

What it means

Error "last argument to ws.connect must be a function" thrown in grafana/k6.

Source

Thrown at internal/js/modules/k6/ws/ws.go:640

}

func parseConnectArgs(state *lib.State, rt *sobek.Runtime, args ...sobek.Value) (*wsConnectArgs, error) {
	// The params argument is optional
	var callableV, paramsV sobek.Value
	switch len(args) {
	case 2:
		paramsV = args[0]
		callableV = args[1]
	case 1:
		paramsV = sobek.Undefined()
		callableV = args[0]
	default:
		return nil, errors.New("invalid number of arguments to ws.connect")
	}
	// Get the callable (required)
	setupFn, isFunc := sobek.AssertFunction(callableV)
	if !isFunc {
		return nil, errors.New("last argument to ws.connect must be a function")
	}

	headers := make(http.Header)
	headers.Set("User-Agent", state.Options.UserAgent.String)
	tagsAndMeta := state.Tags.GetCurrentValues()
	parsedArgs := &wsConnectArgs{
		setupFn:     setupFn,
		headers:     headers,
		cookieJar:   state.CookieJar,
		tagsAndMeta: &tagsAndMeta,
	}

	if common.IsNullish(paramsV) {
		return parsedArgs, nil
	}

	// Parse the optional second argument (params)
	params := paramsV.ToObject(rt)

View on GitHub (pinned to 93accf6570)

When it happens

Trigger: Thrown at internal/js/modules/k6/ws/ws.go:640 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/08466ce78ccdcc07. Report an issue: GitHub.