Because needing to do something regardless of success or failure is incredibly common (e.g. stop a progress bar, re-enable form inputs & buttons). The (promise

Because needing to do something regardless of success or failure is incredibly common (e.g. stop a progress bar, re-enable form inputs & buttons). The (promises) spec is lacking w.r.t. this use-case, and most ajax libraries (and a lot or promise libraries) recognise this by implementing a 'finally' equivalent.