what is difference between success and .done() method of $.ajax

In short, decoupling success callback function from the ajax function so later you can add your own handlers without modifying the original code (observer pattern).

Please find more detailed information from here: https://stackoverflow.com/a/14754681/1049184

success only fires if the AJAX call is successful, i.e. ultimately returns a HTTP 200 status. error fires if it fails and complete when the request finishes, regardless of success.

In jQuery 1.8 on the jqXHR object (returned by $.ajax) success was replaced with done, error with fail and complete with always.

However you should still be able to initialise the AJAX request with the old syntax. So these do similar things:

// set success action before making the request
$.ajax({
  url: '...',
  success: function(){
    alert('AJAX successful');
  }
});

// set success action just after starting the request
var jqxhr = $.ajax( "..." )
  .done(function() { alert("success"); });

This change is for compatibility with jQuery 1.5's deferred object. Deferred (and now Promise, which has full native browser support in Chrome and FX) allow you to chain asynchronous actions:

$.ajax("parent").
    done(function(p) { return $.ajax("child/" + p.id); }).
    done(someOtherDeferredFunction).
    done(function(c) { alert("success: " + c.name); });

This chain of functions is easier to maintain than a nested pyramid of callbacks you get with success.

However, please note that done is now deprecated in favour of the Promise syntax that uses then instead:

$.ajax("parent").
    then(function(p) { return $.ajax("child/" + p.id); }).
    then(someOtherDeferredFunction).
    then(function(c) { alert("success: " + c.name); }).
    catch(function(err) { alert("error: " + err.message); });

This is worth adopting because async and await extend promises improved syntax (and error handling):

try {
    var p = await $.ajax("parent");
    var x = await $.ajax("child/" + p.id);
    var c = await someOtherDeferredFunction(x);
    alert("success: " + c.name);
}
catch(err) { 
    alert("error: " + err.message); 
}

.success() only gets called if your webserver responds with a 200 OK HTTP header - basically when everything is fine.

The callbacks attached to done() will be fired when the deferred is resolved. The callbacks attached to fail() will be fired when the deferred is rejected.

promise.done(doneCallback).fail(failCallback)

.done() has only one callback and it is the success callback