console.log('before');

function async(callback){
    var sum = 10;
    //return callback(null, sum);
    //return process.nextTick(callback.bind(null, null, sum)); // context, err, sum
    //return setImmediate(callback.bind(null, null, sum));
    return setTimeout(callback.bind(null, null, sum), 0);
}

async(function(err, sum){
    if(err) throw new Error('warning');
    console.log('result is : ', sum);
});

console.log('after');