Add withExtraArgument()

[gaearon]

People often request the ability to inject a custom argument into thunks. While we don’t plan to do this by default so that we don’t have to change the signature of redux-thunk, we are adding thunk.withExtraArgument(arg) as a way to inject an arbitrary argument into every thunk action creator. This argument will be available as the third argument in the action creators, right after getState().

Example:

const store = createStore(
  reducer,
  applyMiddleware(thunk.withExtraArgument(api))
)

// later
function fetchUser(id) {
  return (dispatch, getState, api) => {
    // you can use api here
  }
}

I chose withExtraArgument over something more compact like inject to make it clear we’re not mutating redux-thunk itself, and that this argument will be passed as an extra argument (rather than, for example, instead of, the two usual arguments).

[markerikson]

Semi-stupid question: if you're going to add the ability to augment with one extra argument, why not make it multiple extra arguments?

[gaearon]

I don’t see much of a point to that. You can always destructure if you need them:

const store = createStore(
  reducer,
  applyMiddleware(thunk.withExtraArgument({ api, whatever }))
)

// later
function fetchUser(id) {
  return (dispatch, getState, { api, whatever }) => {
    // you can use api here
  }
}

Just one argument seems conceptually simpler than having to explain that the first two arguments are from Redux, and any number of next arguments are your custom ones.

[markerikson]

Okay, yeah, that seems pretty viable. Nice.

[villesau]

Could someone open up why this was implemented? I can't come up with any valid reason/use case why to use this over es6 modules.

[gaearon]

@villesau Useful for mocking an API client in tests, for example.

[villesau]

thanks @gaearon! Is testing the main reason for this or is there some other use cases as well? I'm just curious. We are using rewire to mock the modules in tests so i didn't even think about that.

[gaearon]

Testing and being able to provide a separate pre-initialized API client instance for every request in server rendering.