Sync: compare locations by structure

[smoogly]

On first load history.transitionTo is called
even if state in store matches current history state
when selectLocationState doesn't preserve object reference
(for example using Immutable.fromJS(...).toJS(...)).

[timdorr]

Unfortunately, this isn't what we want to test on. We want to ensure we're dealing with the same location instance. Also, this would cause issues with state being set to something non-serializable.

[smoogly]

My thinking behind this is: selectLocationState is user-supplied method. There's no guarantee it's going to preserve references.

Which is exactly the case in this project, for example: https://github.com/FullstackTypeScript/redux-bootstrap/blob/master/src/index.tsx#L69
(value is passed through Immutable.js; see this pen: https://codepen.io/anon/pen/dREXbX?editors=0010)

When initialLocation isn't equal to locationInStore by reference, extra history transition happens on first load and pollutes browser history (back button stops working properly).

So, may I ask you to clarify: is it the fix incorrect or the problem should never be fixed in its entirety?

[smoogly]

@timdorr, do you think being able to provide comparison function (which would default to reference comparison) is ok?
This way if selectLocationState for some reason doesn't preserve references, it will be possible to specify custom comparison to resolve this.