Readfile to variable react native fs

Hello i have a simple problem - how to get content of some file as String and assign to a String variable?
It always return promise but i cant access other methods or functions from outside which i need cause i need to work with that text inside and use functions that depends on text inside. Do u have some example code how to do it? Is it possible in react native? Could someone explain that to me? I m stuck in my project.