What causes "evaluating JS: %w" in grafana/k6?

Run the same expression in the browser DevTools console first to prove it evaluates cleanly.

evaluating JS: %w

Error message

evaluating JS: %w

What it means

Frame.EvaluateWithContext() waits for the main-world execution context and calls f.evaluate with forceCallable and returnByValue. This error wraps any failure: the evaluated function threw an exception in the page, an argument failed to serialize, the context was destroyed by navigation, or a CDP protocol error occurred.

Source

Thrown at internal/js/modules/k6/browser/common/frame.go:868

	return nil
}

// EvaluateWithContext will evaluate provided page function within an execution context.
// The passed in context will be used instead of the frame's context. The context must
// be a derivative of one that contains the sobek runtime.
func (f *Frame) EvaluateWithContext(ctx context.Context, pageFunc string, args ...any) (any, error) {
	f.log.Debugf("Frame:EvaluateWithContext", "fid:%s furl:%q", f.ID(), f.URL())

	f.waitForExecutionContext(mainWorld)

	opts := evalOptions{
		forceCallable: true,
		returnByValue: true,
	}
	result, err := f.evaluate(ctx, mainWorld, opts, pageFunc, args...)
	if err != nil {
		return nil, fmt.Errorf("evaluating JS: %w", err)
	}

	applySlowMo(ctx)

	return result, nil
}

// Evaluate will evaluate provided page function within an execution context.
func (f *Frame) Evaluate(pageFunc string, args ...any) (any, error) {
	f.log.Debugf("Frame:Evaluate", "fid:%s furl:%q", f.ID(), f.URL())

	return f.EvaluateWithContext(f.ctx, pageFunc, args...)
}

// EvaluateGlobal will evaluate the given JS code in the global object.
func (f *Frame) EvaluateGlobal(ctx context.Context, js string) error {
	action := runtime.Evaluate(js).WithAwaitPromise(true)

View on GitHub (pinned to 93accf6570)

Solutions

Run the same expression in the browser DevTools console first to prove it evaluates cleanly.
Pass arguments via args..., not string interpolation into pageFunc.
Wait for the page/element state the expression depends on (waitForSelector/waitForLoadState).

Example fix

// before
frame.evaluate(`(${id}) => document.getElementById(${id}).value`); // id interpolated unquoted -> syntax error

// after
frame.evaluate('(id) => document.getElementById(id).value', 'username');

Defensive patterns

Strategy: try-catch

Validate before calling

// prove the snippet is valid JS before the run
new Function('(arg) => { ' + pageFuncBody + ' }'); // throws at build time on syntax errors
await page.waitForLoadState('domcontentloaded');

Try / catch

try {
  const v = frame.evaluate(pageFunc, ...args);
} catch (e) {
  console.warn('evaluate failed:', e.message);
}

Prevention

Prototype every pageFunc in DevTools first.
Pass data through args, never interpolation.
Depend waits on the DOM state the snippet assumes.

When it happens

Trigger: pageFunc string is not valid JS or throws at runtime (undefined variable, null deref); args contain non-serializable values; page navigates mid-evaluate; frame detached.

Common situations: Passing a JS snippet that references DOM nodes not yet present; mixing quoted strings into the function string (breaking syntax); evaluating during redirects; passing functions or complex objects as args.

Related errors

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/0c388c2f1106eeec. Report an issue: GitHub.