screenm0nkey screenm0nkey - 10 months ago 285
jQuery Question

jQuery deferreds and promises - .then() vs .done()

I've been reading about jQuery deferreds and promises and I can't see the difference between using

for successful callbacks. I know Eric Hynds mentions that
map to the same functionality but I'm guessing so does
as all the callbacks are all invoked on a completion of a successful operation.

Can anyone please enlighten me to the correct usage?

Many thanks


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.

Prior to jQuery 1.8, then() was just syntactic sugar:

promise.then( doneCallback, failCallback )
// was equivalent to
promise.done( doneCallback ).fail( failCallback )

As of 1.8, then() is an alias for pipe() and returns a new promise, see here for more information on pipe().

success() and error() are only available on the jqXHR object returned by a call to ajax(). They are simple aliases for done() and fail() respectively:

jqXHR.done === jqXHR.success === jqXHR.error

Also, done() is not limited to a single callback and will filter out non-functions (though there is a bug with strings in version 1.8 that should be fixed in 1.8.1):

// this will add fn1 to 7 to the deferred's internal callback list
// (true, 56 and "omg" will be ignored)
promise.done( fn1, fn2, true, [ fn3, [ fn4, 56, fn5 ], "omg", fn6 ], fn7 );

Same goes for fail().