grafana/k6 · error

predicate must be a function

Error message

predicate must be a function

What it means

Error "predicate must be a function" thrown in grafana/k6.

Source

Thrown at internal/js/modules/k6/browser/browser/page_mapping.go:1086

	if k6common.IsNullish(opts) {
		return ropts, nil, nil
	}

	if fn, ok := sobek.AssertFunction(opts); ok {
		return ropts, fn, nil
	}

	obj := opts.ToObject(k6ext.Runtime(ctx))

	var pred sobek.Callable
	for _, k := range obj.Keys() {
		switch k {
		case "timeout":
			ropts.Timeout = time.Duration(obj.Get(k).ToInteger()) * time.Millisecond
		case "predicate":
			fn, ok := sobek.AssertFunction(obj.Get(k))
			if !ok {
				return nil, nil, fmt.Errorf("predicate must be a function")
			}
			pred = fn
		default:
			return nil, nil, fmt.Errorf("waitForEvent does not support option: '%s'", k)
		}
	}

	return ropts, pred, nil
}

// parseSize parses the size options from a Sobek value.
func parseSize(rt *sobek.Runtime, opts sobek.Value) (*common.Size, error) {
	size := &common.Size{}
	if k6common.IsNullish(opts) {
		return size, nil
	}

	obj := opts.ToObject(rt)

View on GitHub (pinned to 93accf6570)

When it happens

Trigger: Thrown at internal/js/modules/k6/browser/browser/page_mapping.go:1086 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/1aa4936d2fcfba07. Report an issue: GitHub.