How to get callback value in node.js in a variable

this is my code.

var fs = require('fs')
var test = readafile('file.txt', function(returnValue) { 
    console.log(returnValue); 
    test = returnValue;
});

console.log(test);

function readafile(filepath,callback){
    var attachment_path = filepath;
    fs.readFile(attachment_path, function(err,data){
        var attachment_encoded = new Buffer(data, 'binary').toString('base64');
        callback(attachment_encoded);
    }); 
}

In that if i need that return value of that function in variable test means how to achieve that ?

In that console.log(test) it says undefined. since it is a callback function. How to get it properly ?