How to display placeholders in AngularJS for undefined expression values?

Question

If I have an expression {{ x }} and x is undefined or null, then how can I display a placeholder for it?

I provided one solution

Answer 1

{{ counter || '?'}}. Just pure javascript. || can be used as default value. Since it would be different empty messages in each, a generalized directive would not be suitable for many cases.

If you want to apply a different class to empty ones, that's also built-in: