[morgante]

> I'm not from the US and don't know the US employment law, but you generally don't have to ask permission from your employer on things you do in your free time.

Employment contracts for developers in the US, particularly at big companies, are very broad and try to lay claim to any development work you do (regardless of whether it's in your "free time" or not). They often expressly forbid freelancing or open source work unless it is approved by the company.

[pjmlp]

And is it tested in court?

In most European countries those type of clauses are anyway invalid, even if written in the contract, as the company doesn't have a say on how you are supposed to live your private life.

Doing freelancing is another matter.