I have a simple case

  .end(function(err, res) {
        assert(err == null,'Error is not null => '+ err.stack);

the problem is of course, if err is null, the assert message will still get evaluated, so it will throw a new/preemptive error saying

Uncaught TypeError: Cannot read property 'stack' of null

I know assert is stable and not likely to change, but the feature I am looking for is somthing like this

assert(err == null, function(){
  return 'Error is not null => '+ err.stack;
});

this of course means, that the callback will only be evaluated if the assertion fails

hope that makes sense, thanks!