combineReducer should give warning if a reducer is not a function

[baibhavx]

Do you want to request a feature or report a bug?*

• Feature

What is the current behavior?

• If the object supplied to combineReducer has a key whose value is not a function, it simply ignores it.

const rootReducer = combineReducers({ 
  todos: todosReducer, 
  steps: {} // no warning, this is simply ignored 
})

What is the expected behavior?

• It should give a warning, letting the user know that each key should point to a reducer function.

Please check out this pull-request with proposed changes: #3026

I will be more than happy to write a test case for this feature.

[timdorr]

I actually found the reason we can't do that: #124

Unfortunately, because of how Babel can transpile modules, you easily end up with extraneous properties going into your export object (e.g., import * as Reducers from './reducers' will have an _esModule property). While not a problem by the specification, it's a practical issue users will run into.

We had the exact same issue with #2279 and had to back out those changes too. Unfortunately, this is still something we need to handle.