once again: done vs then

Question

Hello guys.

I thought I understood the difference between done and then, but now I'm having some doubts about the exception handling. The docs say the following:

Unhandled exceptions in a then function are silently captured as part of the state of the promise, but unhandled exceptions in a donefunction are thrown. Both functions can handle exceptions that have been passed to them as part of the state of a promise.

However, I've noticed that if you don't pass an error method to the then, then the runtime will end up throwing the exception. so, can someone explain (probably with a simple example) what the previous setence means?

thanks.

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Luis Abreu

Answer 1

just putting it on the top again :)

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Luis Abreu

Answer 2

can someone help?

thanks.

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Luis Abreu

Answer 3

Then returns a Promise whereas Done does not. That is the difference.  Done throws exceptions because it is just like any old function.

Answer 4

Hi Luis. 

You can chain up your THEN , and end the chain with a Done.

I believe the behaviour is , exception will be thrown if you have no Done after the last Then.Reason being , if developers don't use Done , then they will never see the exception. I think the doc is just there to encourage people to use Done.

So yes, the document is not accurate in this case.

Answer 5

hello guys.

The problem here is not getting a reference to a Promise. the thing is that the docs say that then will swallow the exception. As I said, I've noticed that there are some times where then does not swallow the exception.

I think I've understood the problem, but can't comment on it yet. lets wait for an official answer from MS.

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Luis Abreu

Answer 6

Done won't swallow an exception. Only the functions that return a promise swallow exceptions.

Answer 7

Yes, that is true. the same happens when you use then (at least, with a some types of promise)

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Luis Abreu