the US has a law that requires the work place to give you a discreet place to pump & they must give you a break to pump when needed. I would look into specific laws for your state. pump & leave milk for baby because switching back & forth can cause baby to be extra gassy & cause your supply to drop or completely dry up

If you have time (I know it's hard) try to pump between feeds before going back to work that way baby can still have breast milk. If you can't then you can use formula, buuuut as @mychellemabelle said, your supply could dip because your body will think it doesn't need to keep producing milk. It's an awful cycle. When you go back to work you'll have to ask for pumping breaks. Some employers will be shitty about it, and depending on your state they might not have to accommodate you, but please look online and check the rules for your state and see if you can get any extra breaks while on the job.

If you don't pump during the day your supply will drop!

I used to baby sit a little boy who had both it was hard at first when his mom was gone but he got used to it. He still does both and he is 9 months