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();

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();

• javascript
• node.js
• promise
• q

• What arguments are you passing to .then, .finally and .done? – Bergi Commented Sep 25, 2014 at 12:00
• I have read the docs, and did not find the answer to be so obvious myself. – Brian Moeskau Commented Sep 26, 2014 at 5:13
• promise is now part of the JS spec, part of the ECMAScript 2015 (ES6) standard . – Pieter Commented Jul 17, 2015 at 9:12

2 Answers 2

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.

The difference is in chaining and error handling, and error logging:

Q(...)
.then(...) 
.finally();

Here, if the then throws, the finally will still run, but no error will log. In Q finally is run regardless of the .then being successful or not. This is like the finally keyword in JS try/catch/finally. It is also possible to chain additional thens to the chain in this case.

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

Here, done indicates that the promise chain has ended, you can not chain to it any more. If you pass it only an onFulfilled handler it will not run if the then threw, and it will log errors if it ends with an exception.