Well sure, but how on earth are you going to claim it does not compete when you use that very code for the employer? By all means try stuff out with some hobby

Well sure, but how on earth are you going to claim it does not compete when you use that very code for the employer? By all means try stuff out with some hobby project but don't be an idiot and tell your employer you've reused 'their' code (or at least, code in their codebase) for other clients. Either get an agreement up front or keep it secret. A contract that grants your employer copyright to code you wrote and used in their codebase is easily enforceable. An exception would be code you wrote before the contract, but in that case using the code without some kind of agreement up front is still dangerous.