My apologies for contradicting you, but this is too simplistic an answer. The answer depends on the laws in your state, whether it was coded on your own time, whether it was coded with any resources of their company, whether ideas in your product can be shown to possibly have come from the work you did for the company, and what your NDA/employment letter/employee agreement states. Unless you run that document by a lawyer and get specific set asides carved out, they might very well be able to take you to court. Some states are really good for you though, like CA. FWIW, I am not a lawyer, and this does not constitute legal advice.

Exactly. We never spent time on the company's dime working on it and my employer actually knew I was working on a side project and permitted me to continue doing so.

And if you are not creating anything related to your work. Like if you work at a game company and you make a game on your own time with your own equipment, the game company you work for can claim your work as their own.