What is the difference between .catch and .fail in jQuery?

catch and fail are slightly different, in that catch will return a new (resolved) promise, whereas fail will return the original promise.

// This will only output "fail"$.Deferred()  .reject(new Error("something went wrong"))  .fail(function() {    console.log("fail");  })  .then(function() {    console.log("then after fail");  })

// This will output "catch" and "then after catch"$.Deferred()  .reject(new Error("something went wrong"))  .catch(function() {    console.log("catch");  })  .then(function() {    console.log("then after catch");  })

Note that catch(fn) is an alias of then(null, fn).

So I think the major difference is what you get out of each.

A catch allows you to run a single function.

A fail allows you to run a number of functions.

Other than that I concur with your findings. They are very similar.

I added an example code to showcase how fail will run both functions, and the catch will only run the one.

 $.ajax({            url: "abc"        }).done(function (data) {        }).fail(function () {            alert("a");        }, function () {            alert("b");        })                .catch(function () {                    alert("c");                }, function () {                    alert("d");                });

If you run this you get 'a','b','c' and then 'd' doesn't run.

I hope this simple example showcases the difference.