grafana/k6 · error

the argument to filter() must be a function, a selector or a

Error message

the argument to filter() must be a function, a selector or a selection

What it means

Selection.Filter() in k6's HTML module accepts exactly three argument kinds: a CSS selector string, another Selection, or a predicate function. The Go code first tries the string and Selection cases via a type switch, then falls back to sobek.AssertFunction; anything that is none of the three throws this error.

Source

Thrown at js/modules/k6/html/html.go:393

			common.Throw(s.rt, fmt.Errorf("the function passed to each() failed: %w", err))
		}
	}

	return Selection{s.rt, s.sel.Each(fn), s.URL}
}

func (s Selection) Filter(v sobek.Value) Selection {
	switch val := v.Export().(type) {
	case string:
		return Selection{s.rt, s.sel.Filter(val), s.URL}

	case Selection:
		return Selection{s.rt, s.sel.FilterSelection(val.sel), s.URL}
	}

	sobekFn, isFn := sobek.AssertFunction(v)
	if !isFn {
		common.Throw(s.rt, errors.New("the argument to filter() must be a function, a selector or a selection"))
	}

	return Selection{s.rt, s.sel.FilterFunction(s.buildMatcher(v, sobekFn)), s.URL}
}

func (s Selection) Is(v sobek.Value) bool {
	switch val := v.Export().(type) {
	case string:
		return s.sel.Is(val)

	case Selection:
		return s.sel.IsSelection(val.sel)

	default:
		sobekFn, isFn := sobek.AssertFunction(v)
		if !isFn {
			common.Throw(s.rt, errors.New("the argument to is() must be a function, a selector or a selection"))
		}

View on GitHub (pinned to 93accf6570)

Solutions

1. Use a selector string: sel.filter('.active')
2. Or a function: sel.filter((idx, el) => el.attr('data-ok') === '1')
3. Or wrap an existing selection: sel.filter(otherSelection)

Example fix

// before
selection.filter(someElement);

// after
selection.filter((idx, el) => el.id() === someElement.id());

const isFilterArg = (v) =>
  typeof v === 'string' ||
  (v !== null && typeof v === 'object' && 'sel' in v && typeof v === 'object' && typeof v.sel !== 'undefined') ||
  typeof v === 'function';
if (!isFilterArg(arg)) throw new TypeError('filter() expects selector, Selection, or function');
sel.filter(arg);

When it happens

Trigger: Calling filter() with a DOM Element (e.g. an item obtained from each()), a number, null/undefined, or an array of nodes.

Common situations: Passing an element handle where a selector or function was intended; calling filter() with no arguments; mixing up the k6/html API with browser-module Element APIs.

Related errors

• the argument to each() must be a function
• the argument to is() must be a function, a selector or a sel
• the argument to map() must be a function
• Error while parsing selector `${selector}`: ${e.message}
• Error while parsing selector `${selector}` - cannot use ${op

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/2c9b977bef89ee87. Report an issue: GitHub.