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document assertReducerSanity behavior #1633

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document assertReducerSanity behavior #1633

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2 changes: 1 addition & 1 deletion docs/api/combineReducers.md
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Expand Up @@ -45,7 +45,7 @@ Any reducer passed to `combineReducers` must satisfy these rules:

* If the `state` given to it is `undefined`, it must return the initial state for this specific reducer. According to the previous rule, the initial state must not be `undefined` either. It is handy to specify it with ES6 optional arguments syntax, but you can also explicitly check the first argument for being `undefined`.

While `combineReducers` attempts to check that your reducers conform to some of these rules, you should remember them, and do your best to follow them.
While `combineReducers` attempts to check that your reducers conform to some of these rules, you should remember them, and do your best to follow them. `combineReducers` will check your reducers by passing `undefined` to them; this is done even if you specify initial state to `Redux.createStore(combinedReducers(...), initialState)`. Therefore, you **must** ensure your reducers work properly when receiving `undefined` as state, even if you never intend for them to actually receive `undefined` in your own code.

#### Example

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