It is similar to a Monad, but not quite, because the "bind" (addCallback) mutates it inplace, and does not return a chained operation. Also because if your callback's return value is itself again a Deferred, it will flatten it out. That means it "auto-joins" on the result so if you want to actually result in some Deferred object in your callback you have to wrap it to circumvent the flattening.