grafana/k6 · error

"handler" argument cannot be nil

Error message

"handler" argument cannot be nil

What it means

Page.On (page.go:1437-1446) registers a handler for page events; currently 'console' is the only accepted event. A nil handler is rejected immediately with '"handler" argument cannot be nil' — there is nothing sensible to invoke on the event.

Source

Thrown at internal/js/modules/k6/browser/common/page.go:1442

	ConsoleMessage *ConsoleMessage

	// Metric is the metric event event.
	Metric *MetricEvent

	// Request is the read only request that is about to be sent from the
	// browser to the WuT.
	Request *Request

	// Response is the read only response that was received from the WuT.
	Response *Response
}

// On subscribes to a page event for which the given handler will be executed
// passing in the ConsoleMessage associated with the event.
// The only accepted event value is 'console'.
func (p *Page) On(event PageEventName, handler PageEventHandler) error {
	if handler == nil {
		return errors.New(`"handler" argument cannot be nil`)
	}

	_, err := p.addEventHandler(event, handler)
	return err
}

func (p *Page) addEventHandler(event PageEventName, handler PageEventHandler) (id uint64, err error) {
	p.eventHandlersMu.Lock()
	defer p.eventHandlersMu.Unlock()

	r := pageEventHandlerRecord{
		id:      p.eventHandlerLastID.Add(1),
		handler: handler,
	}
	p.eventHandlers[event] = append(p.eventHandlers[event], r)

	return r.id, nil
}

View on GitHub (pinned to 93accf6570)

Solutions

1. Always pass a function as the second argument to page.on()
2. If the handler is optional in your harness, default it to a no-op: page.on('console', handler || (() => {}))
3. Check for typos in the handler variable name before registering

Example fix

// before
page.on('console');

// after
page.on('console', (msg) => console.log(msg.text()));

// Guard before registering
const handler = typeof cb === 'function' ? cb : () => {};
await page.on('console', handler);

const isPageEventHandler = (h) => typeof h === 'function';

When it happens

Trigger: page.on('console') without a second argument; passing a null/undefined variable as the handler (e.g. a callback assigned conditionally); destructuring or renaming that leaves the handler undefined.

Common situations: Copy-paste from docs that drops the callback; optional handlers wired only when a flag is set; refactors that rename the callback function.

Related errors

• predicate function is not callable
• clip area is either empty or outside the viewport
• provided selector is empty
• only one of the selectors can capture using * modifier
• unknown browser event: %q, must be %q

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/c0e3a0ac3ca1ab43. Report an issue: GitHub.