Store: Avoid Lodash get for getPostEdits

[aduth]

This pull request seeks to refactor the getPostEdits selector to avoid relying on Lodash's _.get. The _.get function includes some overhead, and there is never a case in the runtime application where the state.editor.present.edits value does not exist, since it is part of the accumulated reducer shape. It was used only for convenience in tests, which is not a justifiable reason to introduce the overhead.

Testing instructions:

Verify unit tests pass:

npm run test-unit editor/store/test/selectors.js

[tofumatt]

This makes sense and is a definite improvement.

It highlights the risks of creating a raw state object in tests rather than dispatching actions to create a usable state. We can into this a lot on the addons-frontend project and created test helpers to initialise actual states rather than making up fake ones.

Is that something we could do for Gutenberg's tests? If yes I'll file a code quality issue, I just want to make sure it wasn't avoided for a technical reason...

[aduth]

@tofumatt This is an excellent point. It's come up a few times informally in the past. I agree it's not great that we have to manually craft the state shape when testing selectors, and it's been particularly painful when that state shape changes (and its resulting impact on selector tests).

I'd considered something similar to what you propose, where we could effectively just run the store reducer and merge in the tree paths (via _.merge) the values relevant for the test case.

On the other hand, the one benefit to manually crafting state shape is that if in-fact these selectors depend on parts of the state shape existing, even if indirectly, it's important that these are considered in their own test cases. For example, it can be argued that our unit tests for isPermalinkEditable are incomplete because they do not include a case where an edits value for permalink_template takes precedent over the one for the saved post (by its use of getEditedPostAttribute). Conversely, if we don't want the edited value to be considered and instead only that of the saved post, the selector should use getCurrentPost instead of getEditedPostAttribute. This becomes obvious here only in not allowing the state.editor.present.edits path to be unreachable.

I agree it'd be good to create an issue to discuss this further.