jquery - Return backup when deferred fails -

we familiar the $.deferred() behaviour when succeeds , fails:

function foo() {     var backup = 'bar',          dfd = $.ajax(...)         .done(function(data, textstatus, jqxhr) {             alert(data);         })         .fail(function(jqxhr, textstatus, errorthrown) {             alert(errorthrown);         });      return dfd.promise(); }  // outside function $.when(foo())     .always(function(something) {         // 'something' either data, or jqxhr, depending if thing fails     }); 

however, have backup result of data, known backup, residing inside function foo, i'd return when request fails.

provided can neither change parameters set in $.ajax(...) (meaning cannot add "fail" handler), nor change return type of foo, nor move backup outside foo, how can achieve following effect?

function foo() {     var backup = 'bar',         dfd = $.ajax(...)         .done(function(data, textstatus, jqxhr) {             alert(data);         })         .fail(function(jqxhr, textstatus, errorthrown) {             // replace return 'bar', impossible              // because 'data' undefined             data = backup;         });      return dfd.promise(); }  // outside function $.when(foo())     .always(function(something) {         // 'something' fresh data, or 'bar' if ajax fails     }); 

create own deferred object, instead of using 1 returned $.ajax():

function foo() {     var def = $.deferred();     var backup = 'bar';     $.ajax(...)         .done(function(data, textstatus, jqxhr) {             def.resolve(data);         })         .fail(function(jqxhr, textstatus, errorthrown) {             def.resolve(backup);         });      return def.promise(); } 

...

foo().done(function(data) {  });