WIP Pass extra argument to actions

[lohfu]

This PR implements the option to initialize a store with an extra argument that will be passed to each invocation of action functions. This is very similar to the extra argument option in redux-thunk#injecting-a-custom-argument.

This branch is based on my new actions branch/PR. The other branch should be merged before this as implementing this functionality on the current actions logic would get kind of confusing.

const store = createStore({}, api)

const action = (query) => async (state, store, api) => ({
  stuff: await api(query),
})

// multiple arguments

const store = createStore({}, { api, whatever })

const action = (query) => async (state, store, { api, whatever }) => ({
  stuff: whatever(await api(query)),
})

Why?

Isomorphic actions / SSR

There are several use cases, an obvious one is passing different fetch implementations or similar
depending on if it is server or client side rendered.

// frontend
import rek from 'rek'

const store = createStore({}, rek)

// server
import rek from 'rek'

const store = createStore({}, rek.extend({ baseUrl: 'http://localhost:1337' }))

// isomorphic action
export const fetchCars = () => (state, store, rek) => (
  rek('/api/cars').json().then(cars => ({ cars }))
)

Easier testing

If the action uses browser globals, they have to be mocked:

export const fetchCars = () => (state, store) => (
  fetch('/api/cars').then(res => res.json()).then(cars => ({ cars }))
)

To test the following either a mock server has to be running, or something like proxyquire be used:

import fetch from 'node-fetch'

export const fetchCars = () => (state, store) => (
  fetch('/api/cars').then(res => res.json()).then(cars => ({ cars }))
)

Both problems above are solved by passing in a fetch argument. Then we simply pass in a mock fetch when testing:

export const fetchCars = () => (state, store, fetch) => (...)

Build Size

	full/preact.js	dist/unistore.js	preact.js
master	760B	355B	546B
feature/new-action-creators	737B	327B	558B
feature/extra-arguments	738B	328B	558B