Integrating logs to testing.T instance.

[stroiman]

When creating a browser.Browser, you have the capability to inject an slog.Logger. You can easily inject a logger that will forward browser logs to the specific test instance, keeping relevant log output with failing tests.

This actually helped find a bug in Gost-DOM.

The bug was elusive as through setTimeout() or friends don't propagate errors back to test code. But errors will be logged on the logger. But a JS error would normally indicate the system isn't doing what you think it should, and therefore a failing test would make sense.

Gost-DOM will by default generate Error level logs on

• JavaScript errors
• 5xx Status codes ¹
• Network errors performing HTTP requests.

4xx status errors generates Warning level logs.²

In my own test project, I created TestingLogHandler a valid implementation of slog.Handler that pass logs to the testing.T/B instance; and triggers an Error report on Error level logs.

The allowErrors provide escape hatch for cases where errors are allowed to occur; which I use in CSRF tests. HTMX throws on the HTTP responses; and I didn't prioritize UX for failed validation.

type TestingLogHandler struct {
	testing.TB
	allowErrors bool
}

func (l TestingLogHandler) Enabled(_ context.Context, lvl slog.Level) bool {
	return lvl >= slog.LevelInfo
}
func (l TestingLogHandler) Handle(_ context.Context, r slog.Record) error {
	l.TB.Helper()
	if r.Level < slog.LevelError || l.allowErrors {
		l.TB.Logf("%v: %s", r.Level, r.Message)
	} else {
		l.TB.Errorf("%v: %s", r.Level, r.Message)
	}
	return nil
}

func (l TestingLogHandler) WithAttrs(attrs []slog.Attr) slog.Handler { return l }
func (l TestingLogHandler) WithGroup(name string) slog.Handler       { return l }

func TestServer(t *testing.T) {
	handler := TestingLogHandler{ TB: t }
	browser := browser.New(
		browser.WithHandler(serv),
		browser.WithLogger(slog.New(handler)),
	)
}

This approached helped find a bug in Gost-DOM itself. HTMX swapping into an empty element failed, because the V8 version of insertBefore generated an error if only one argument was passed; the correct behaviour would have been to call the underlying Go version with nil as the 2nd argument

Log strategy

In general, it the intention is that Gost-DOM logs at the following levels.

• Debug
  • Native functions called from JavaScript.
  • JavaScript console.debug / console.trace (trace not supported yet)
• Info
  • HTTP requests from XHR or normal navigation
  • HTTP status codes < 300
  • Events dispatched by EventTarget
  • JavaScript console.log and console.info
• Warning
  • HTTP 4xx Status codes
  • JavaScript console.warn
• Error
  • HTTP 5xx status codes
  • JavaScript runtime errors
  • JavaScript console.error
  • Other error conditions

---

Footnotes

1. At the time of writing this, non 2xx errors generate Error level logs - but the statement will be true from next release. ↩

2. There is not "contract" for the contents of the structured log record; but I'd love to get feedback as to what kind of format would make sense. ↩