grafana/k6 · error

the argument to map() must be a function

Error message

the argument to map() must be a function

What it means

Selection.Map() in k6's HTML module implements ES5 Array.prototype.map over the matched elements and requires a single function argument; sobek.AssertFunction rejects everything else. Results of the callback are collected into a plain array of sobek values (callbacks that throw are silently skipped, their result omitted).

Source

Thrown at js/modules/k6/html/html.go:421

	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 {
			values = append(values, fnRes)
		}
	})

	return values
}

func (s Selection) Slice(start int, def ...int) Selection {
	// We are forced to check that def[0] is inferior to the length of the array
	// otherwise the s.sel.Slice panics. Besides returning the whole array when
	// the end value for slicing is superior to the array length is standard in js.

View on GitHub (pinned to 93accf6570)

Solutions

1. Pass a callback: sel.map((idx, el) => el.text())
2. For selecting subsets first, chain .find() or .filter() before .map()
3. Confirm the callback variable is defined at call time (typos yield undefined)

Example fix

// before
const texts = selection.map('div');

// after
const texts = selection.find('div').map((idx, el) => el.text());

if (typeof fn !== 'function') throw new TypeError('map() expects a function');
const values = sel.map(fn);

When it happens

Trigger: Calling map() with a selector string, no argument, or a non-callable value; expecting jQuery-style arguments without providing the callback.

Common situations: Attempting sel.map('div') to project elements (should be find + map); refactor that dropped the lambda; passing an arrow function stored in a variable that is actually undefined.

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 is() must be a function, a selector or a sel
• 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/602fdbee886680f3. Report an issue: GitHub.