Add warning on undefined select

lib/potent-reducer.js

@@ -16,22 +16,6 @@ export const usePotentReducer = options => {
   return [state, _thunks]
 }
 
-/** A ReducerStore Factory that uses React.context
- * Used to make a Store accessible through multiple components
- */
-export const createStore = options => {
-  const context = createContext(null)
-  const { Consumer } = context
-  const Provider = ({ children }) => {
-    const store = usePotentReducer(options)
-    return React.createElement(context.Provider, { value: store }, children)
-  }
-  const useStore = () => useContext(context)
-  const useSelector = selector => selector(useContext(context)[0])
-  const useDispatch = () => useContext(context)[1]
-  return { Provider, Consumer, useStore, useSelector, useDispatch }
-}
-
 /** Turn a reducer definition object to a function
  * @param {Object} reducer: Object definition of a reducer: {<type>: (state, payload) => <newState>}
  * @param {Object} options
@@ -86,6 +70,35 @@ function patchDispatch(dispatch, type) {
   }
 }
 
+/** A ReducerStore Factory that uses React.context
+ * Used to make a Store accessible through multiple components
+ */
+export const createStore = options => {
+  const context = createContext(null)
+  const { Consumer } = context
+  const Provider = ({ children }) => {
+    const store = usePotentReducer(options)
+    return React.createElement(context.Provider, { value: store }, children)
+  }
+  const useStore = () => useContext(context)
+  const useSelector = selector => {
+    const val = selector(useContext(context)[0])
+    options.warnOnUndefinedSelect && assertSelectedExists(val)
+    return val
+  }
+  const useDispatch = () => useContext(context)[1]
+  return { Provider, Consumer, useStore, useSelector, useDispatch }
+}
+
+function assertSelectedExists(value) {
+  console.warn(`A selector returned undefined.
+This indicates that you tried to access a state property that does not exist.
+To turn these messages off use {"warnOnUndefinedSelect": false} in options`)
+  console.groupCollapsed('Trace')
+  console.trace()
+  console.groupEnd()
+}
+
 /** Log action inspired by redux-logger */
 const logger = ({ prevState, action, nextState }) => {
   const css = color => `color: ${color}; font-weight: bold;`