Don't entertain the idea of working for free. Your time is valuable. You don't need to work for a company that does not respect your time. If they want a demo

Don't entertain the idea of working for free. Your time is valuable. You don't need to work for a company that does not respect your time. If they want a demo of your work have them look at your Github. If they want more give them an estimate of the time required to write it and what your rate is. Don't let them get away with getting your work for free.