Should `combineReducers` return the same object if nothing changes?

[aaronjensen]

Right now, the reducer returned by combineReducers returns a new object regardless of whether or not the reducers it is combining make any changes. This means breaking reference equal every action, which limits its usefulness when using nested combinations unless you always select all the way down to the piece of state that maintains reference equality integrity. It'd be a little bit more slow to introduce an equality check, after the mapValues but it seems like it'd be better, yeah?

[gaearon]

Let's add the check. Something like hasChanged = false which gets set to true if newState[key] !== state[key]. If !hasChanged, we will return the previous state.

[georgebonnr]

💯 Went through a painful process figuring out why disparate parts of my app were re-rendering. Was about to write my own function when I figured this out, but glad I can update instead. 👍

[Tomassito]

@aaronjensen Can you help me understand the issue here? You're normally interested in only a part of the state that you're combining together. You'd have to have a dependency on the whole combined chunk in order to run into trouble, right? Have you got such usecase really?

[aaronjensen]

I don’t use redux anymore. This issue is 6 years old.

[aaronjensen]

I imagine I did have a use case at the time or I would not have opened an issue and submitted a patch.