Implementation of callback in database request
If you run multiple async functions in a certain orderi it will look like something like this : 

getData(function(a){  
    getMoreData(a, function(b){
        getMoreData(b, function(c){ 
            getMoreData(c, function(d){ 
                getMoreData(d, function(e){ 
                    ...
                });
            });
        });
    });
});


but it does not look nice and hard to control. Instead use promises, I recommend bluebirdjs library If you use nodejs, take it easy.