Whats the difference between Q Promise library's .finally() and .done()?

javascript node.js promise q
Todd Bluhm picture Todd Bluhm · Sep 25, 2014 · Viewed 7.7k times · Source

What is the difference between using the Nodejs Q promise library's .finally() and .done() statements.

For example whats the difference between these two?

Q(...)
.then(...)
.finally(); //or fin()

Q(..)
.then()
.done();

Answer

Mariusz Nowak picture Mariusz Nowak · Sep 25, 2014

promise.done(onSuccess, onError) simply allows you to process resolved value. An additional benefit is that does not imply any error swallowing (as it is the case with promise.then()), it guarantees that any involved exception would be exposed. It also effectively ends the chain and does not return any further promise.

promise.finally(fn) is for registering a task that must be done after a given promise resolves (it doesn't matter whether promise succeeds or fails). Usually, you use it for some kind of cleanup operations e.g. imagine you set up a progress bar, that needs to be hidden after the request is done (no matter if it was successful), then just do promise.finally(hideProgressBar). Additionally promise.finally() returns input promise, so you can return it for further processing.

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