var fib = function (n) {
    if (n < 2) return n;
    return fib(n - 1) + fib(n - 2);
};
var Obj = function() { };
Obj.prototype.doSomething = function(arg1_) {
   var callback_ = arguments[arguments.length - 1];
   callback = (typeof(callback_) == 'function' ? callback_ : null);
   var arg1 = typeof arg1_ === 'number' ? arg1_ : null;
   if (!arg1) return callback(new Error('first arg missing or not a number'));

   process.nextTick(function() {
     // block on CPU
     var data = fib(arg1);
     callback(null, data);
    });
}
var test = new Obj();
var number = 10;
test.doSomething(number, function(err,value) {
      if (err)
         console.error(err);
      else
         console.log('fibonaci value for %d is %d', number, value);
});
console.log('called doSomething');