Callback hell is a phenomenon it creates a lot of problems for a JavaScript developer when tries to execute multiple asynchronous operations one after the other. It is called an asynchronous function when some external activity must completed before processing a result. It is called asynchronous .
Example :
getData(function(a){
getMoreData(a, function(b){
getMoreData(b, function(c){
getMoreData(c, function(d){
getMoreData(d, function(e){
...
});
});
});
});
});