const increment = function(number, callback){
    let result = number + 1;
    if(callback)
        callback(result);
}

// Basic usage 2 + 1 = 3
increment(2, function(res){
    console.log(res);
});

// Callback Hell 2 + 1 + 1 + 1 + 1 + 1 = 7
increment(2, function(res1){
    increment(res1, function(res2){
        increment(res2, function(res3){
            increment(res3, function(res4){
                increment(res4, function(res5){
                    console.log(res5);
                });
            });
        });
    });
});