how to use callbacks according to NPM style guides

callbacks.js

// in your library.js file
module.exports = {
    getFibonacciNumber: function (n, done) {
        if (n < 0) {
            return done("Can only calculate fibonacci numbers for an integer n > -1 !");
        } else {
            var a = 0
              , b = 1
              , i = 0
              , holder
              ;
            
            while (i++ < n) {
                holder = a;
                a = b;
                b = a + b;
            }
            return done(null, b);
        }
    }
}

// somewhere else
var library = require("library")
var nth = 25;
library.getFibonacciNumber(nth, function (err, n) {
    if (err) {
        console.log(err);
    } else {
        console.log("The " + nth + "th Fibonacci number is " + n);
    }
})