combineReducers behavior when state slices return undefined during restoration

State Consistency in Redux Reducers

When implementing state restoration logic in Redux, the behavior of combineReducers regarding undefined return values can impact the integrity of the recovered state tree.

In standard operation, if a reducer returns undefined for a specific key, that key may be omitted from the resulting state object. This creates a discrepancy between the expected schema of a restored backup and the actual state tree managed by the store.

Given that combineReducers expects each reducer to return a valid state slice or its defined default initial state, the uncertainty lies in how the store handles the transition when a restoration action triggers a reducer to return undefined instead of a fallback value.

• Does combineReducers maintain the previous state slice if the reducer returns undefined during a restoration dispatch?
• What is the documented behavior for state tree keys that are omitted due to undefined returns in Redux 4.x and 5.x?