grafana/k6 · error

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

Error message

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

What it means

Selection.Is() in k6's HTML module checks whether the current selection matches a CSS selector, another Selection, or a predicate function, and throws when the argument is none of these. The type switch handles string and Selection; the default branch asserts a function via sobek.AssertFunction before calling goquery's IsFunction.

Source

Thrown at js/modules/k6/html/html.go:410

	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"))
		}

		return s.sel.IsFunction(s.buildMatcher(v, sobekFn))
	}
}

// Map implements ES5 Array.prototype.map
func (s Selection) Map(v sobek.Value) []sobek.Value {
	sobekFn, isFn := sobek.AssertFunction(v)
	if !isFn {
		common.Throw(s.rt, errors.New("the argument to map() must be a function"))
	}

	var values []sobek.Value
	s.sel.Each(func(idx int, sel *goquery.Selection) {
		selection := &Selection{sel: sel, URL: s.URL, rt: s.rt}

		if fnRes, fnErr := sobekFn(v, s.rt.ToValue(idx), s.rt.ToValue(selection)); fnErr == nil {

View on GitHub (pinned to 93accf6570)

Solutions

1. Pass a selector: sel.is('.visible')
2. Or a Selection to test overlap: sel.is(otherSel)
3. Or a function: sel.is((idx, el) => el.attr('aria-hidden') !== 'true')

Example fix

// before
if (selection.is(element)) { ... }

// after
if (selection.is((idx, el) => el.id() === element.id())) { ... }

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

When it happens

Trigger: Calling is() with a DOM Element, null, undefined, a boolean, or any other non-string/non-Selection/non-function value.

Common situations: Comparing a selection against an element variable; forgetting the argument in a quick REPL test; chaining is() after a call that already returned a boolean.

Related errors

• the argument to each() must be a function
• the argument to filter() must be a function, a selector or a
• 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/caa8282ada554dfe. Report an issue: GitHub.