It depends a bit on the jurisdiction (I'm not really sure if laws or local culture came first, but they tend to go hand-in-hand for this issue). E.g., in CA th

It depends a bit on the jurisdiction (I'm not really sure if laws or local culture came first, but they tend to go hand-in-hand for this issue). E.g., in CA there are only a few whitelisted situations where your employer could own your side project, and in all other cases it belongs to you. As a matter of practice, most companies in the area allow side projects, and many "require" that you ask permission -- since they'll just rubber stamp approval onto whatever you're doing anyway, that typically works out in your favor; now you have written proof that your employer doesn't want to exercise any potential claims to your project even if it nominally seems to infringe on their business.