feat(action): support optional error field

[tvanier]

Allow prepareAction to return an optional error field as per the Flux specification
https://github.com/redux-utilities/flux-standard-action#actions

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[markerikson]

Y'know, that's a good point.

It's frankly the least-used part of the FSA spec (most folks just dispatch SOME_ACTION_ERROR instead), but this would be a reasonable thing to include.

Can you add some types tests for this functionality as well?

@phryneas , any thoughts here?

[phryneas]

Looks good to me, but as you said, this still need type-tests.

@tvanier : take a look over here : https://github.com/reduxjs/redux-starter-kit/blob/master/type-tests/files/createAction.typetest.ts

Some additional tests that if prepareAction returns an error, it is also defined on the created Action with the same type. And additionally, just add some // expect-error cases to existing tests, meaning that if no error is returned from prepareAction, no error property is present on the returned action.

[markerikson]

If we can get this in in the next day or two, we can get this into 1.0. Otherwise, we'll just add it afterwards (1.0.1 or something).

[tvanier]

Sorry for having missed some tests, I am not too familiar with type-testing, just added some as suggested by @phryneas , let me know what you think. And please do not block the 1.0 release if this PR is not ready! (although supporting error would help our project)

[phryneas]

added a few comments - unfortunately our type-tests aren't as specific on what they're testing as I'd like them to be everywhere, so they might not have been a very good example

[phryneas]

Please replace this with something like

// typings:expect-error
const x: any = strLenMetaAction('test').error

you want to see that the property does not exist at all, while the

// typings:expect-error 
expectType<boolean>(...)

just ensures that it isn't boolean.

[tvanier]

I updated the test as suggested.
Out of curiosity, would it be possible to add Jest expectations here?

  // typings:expect-error
  const error: any = strLenAction('test').error
  expect(error).toBeUndefined()

[phryneas]

I don't think that code is actually ever run, it should only be compiled to look for compiler warnings. Doing something like this would work better in the normal tests.

We just want to make sure that there's no regression in the types & autocompletion.

[phryneas]

Looks good to me now.

[tvanier]

Thanks @phryneas .
@markerikson It would be great if this could be merged before releasing 1.0, I am adding the starter kit to a project where action.error is used already.

[markerikson]

Cool. Awright, in it goes!

[tvanier]

Great, thanks @markerikson and @phryneas !
Do we have an ETA for 1.0, or 0.10 maybe?

[markerikson]

Coming Soon (TM) :)