How to use properly fs.readfile to store data into a variable?

See more:

Javascript

file-system

Node.js

I'm trying to use fs.readFile() to read the contents of some file and do some operations with the data returned. I'm struggling to store the data into a global variable to use in other functions. In the sample below, how can I "store" the output of the callback function (which I named xxx) in a global variable from the else (the result of the createArray())? To be precise, in this last function createArray() I make later conversions and some operations with the content of the file, but I still don't get it as global but just in the console. Having the value of xxx variable as global would be just enough at the moment. Is it possible to do that with an asyncronous version? Or Am i proceeding in a wrong path ?

function readF(filename){

fs.readFile(filename,{encoding:'utf8', flag:'r'},callback);

}

function callback (err, data) {
if(err)
console.log(err);
else
xxx = createArray(data);
console.log(xxx) // IT IS PRINTED CORRECTLY INSIDE THE FUNCTION
return xxx;
}

console.log(xxx) //PRINTS UNDEFINED

What I have tried:

Tried to store callback function return into a variable, still getting undefined.
Tried with recursion, also didn't work.