Create a "combineStores" utility function to combine stores

Feature Description

Currently, we create “combined” stores like this: https://github.com/google/site-kit-wp/blob/507e81f4af2b52e77db6a71710777d5ea086302b/assets/js/googlesitekit/modules/create-module-store.js#L59

But that’s a bit verbose and makes combining multiple stores together a bit annoying. We should create a utility that combines stores—even ones that lack certain keys.

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Acceptance criteria

• Create a combineStores( …args ) function that accepts a list of stores and combines their actions, reducers, etc. for multiple stores and returns a single store. It should not require that stores have all store properties (eg. initialState, actions, etc.) and should use sane defaults for any missing values.
• It should still fail if duplicate keys are found, similar to the collectActions, etc. functions.

Implementation Brief

• Add a combineStores( …args ) function that accepts full stores as arguments, each with the object shape:

{
  INITIAL_STATE,
  actions,
  controls,
  reducer,
  resolvers,
  selectors,
}

Note that every value should be optional, because it’s possible to have a store that doesn’t have controls, or even selectors, etc. We can provide sane defaults for other things like reducer or initialState if none are provided. (Though equally, I’d be fine if initialState was also required.)

• This should act like an amalgamation of this type of code: https://github.com/google/site-kit-wp/blob/c302d8cb77374e394ba4711a93b3d7ca33e43802/assets/js/googlesitekit/datastore/site/index.js#L38-L85, but allow for developers to simply submit entire stores. So the above code would change to:

const store = Data.combineStores(
	{
		actions: Data.commonActions,
		controls: Data.commonControls,
	},
	connection,
	info,
	reset,
	notifications,
);

Data.registerStore( STORE_NAME, store );

QA Brief

Changelog entry

• Add googlesitekit.data.combineStores utility function to combine multiple datastore objects.

[felixarntz]

IB ✅

Let’s have all parts of the object optional including INITIAL_STATE and use defaults (INITIAL_STATE could just be an empty object).

[felixarntz]

@jqlee85 Yeah exactly, sorry for being a bit unprecise in my comment above. combineStores should not use any initialization logic indirectly either 👍 The actual problem as mentioned lies in addInitializeAction being called with an empty object for commonStore.