I'm not saying that a backend developer can't do some frontend, or that a frontend developer can't do some backend. I'm just saying that the mythical "full stack developer" that is as good as a dedicated backend developer and dedicated frontend developer put together simply doesn't exist. Any company that thinks this mythical creature exists is going to be sorely disappointed. Rather say that you're looking for a backend developer who knows some frontend, or a frontend developer who knows some backend. Otherwise you're going to land up with a jack of all trades and master of none.