Make middleware API dispatch pass through all call arguments

[Asvarox]

As discussed in #2501, there's a bit of inconsistency as you can call store.dispatch with multiple arguments just fine, but in context of middlewares (say, thunk) only the first one is passed through. This PR tries to fix that.

[timdorr]

This should be fine. It definitely isn't breaking. Middleware can still block other args from being passed through to next(), but it at least opens up the possibility to do so.

[gaearon]

I think the reason we didn't do this initially is because we wanted to make it easier for all middleware to be compatible. Now we need to either force all middlewares to pass it through or live with a situation where adding a seemingly harmless middleware can break the other code in a somewhat non-obvious way.

[gaearon]

As for the use case in #2501, I don't quite see what prevents the user from passing an array: dispatch([myThunk, arg1, arg2]).

[timdorr]

Middleware changes the definition of what dispatch's args can be anyways. So whether you use an array or allow multivariate functions, it's really no different because the function signature is changed. It's odd for a loosely-designed API to have a restriction thrown in there.

[markerikson]

@timdorr : I'd disagree with the phrase "changes the definition of what dispatch's args can be", at least to some extent. Yes, they allow you to pass in something other than a plain action, but the working assumption thus far has been that there's only one value being passed through, much like a promise pipeline.

As you said, allowing multiple args here shouldn't break any existing behavior, but given that all existing middlewares assume there's only one arg, I'm not sure about the actual benefit. (This is closely related to the very long discussion in #1813 .)

[gaearon]

IMO allowing this will benefit a tiny minority of users (who already have a fine workaround) but will add cost to every middleware author who will eventually be asked to support this. This change, to be successful, will have to trigger a cascade of changes in every library that delegates to dispatch. And it makes Redux harder to type (which is already hard). In my eyes the cost outweighs the benefit.

[sergeysova]

why not just return dispatch?

const middlewareAPI = {
  getState: store.getState,
  dispatch,
}

[FranciscoMSM]

Hey! Can someone have an explanation for this change?

[timdorr]

@FranciscoMSM This particular change is to allow middleware to change the arguments of dispatch and pass that through.

The reason for the extra function is to remove direct access to dispatch so middleware doesn't try to attach properties or other hidden metadata to it. Each middleware gets its own unique copy of dispatch this way, and there is less chance of interference between middlewares.