The vast majority of your code probably isn't manipulating state, or doesn't need to be. You can have well defined boundaries where you're dealing with state.

The vast majority of your code probably isn't manipulating state, or doesn't need to be. You can have well defined boundaries where you're dealing with state. Read from the database, pass those values through a bunch of pure functions, then eventually rewrite to the database. You only have side effects when you intially read from and eventually write to the DB. Same goes for manipulating the DOM, and almost anything else.