Throw helpful error when router reducer not mounted under "router"

src/selectors.js

@@ -1,9 +1,15 @@
 import { matchPath } from "react-router"
+import isObject from 'lodash/isObject'
 
 const createSelectors = (structure) => {
-  const { getIn, toJS } = structure
-  const getLocation = state => toJS(getIn(state, ['router', 'location']))
-  const getAction = state => toJS(getIn(state, ['router', 'action']))
+  const { get, toJS } = structure
+  const getRouter = state => {
+    const router = toJS(get(state, 'router'))
+    if (!isObject(router)) { throw 'Could not find router reducer in state tree. Are you sure it is mounted under "router"?' }
+    return router
+  }
+  const getLocation = state => toJS(get(getRouter(state), 'location'))
+  const getAction = state => toJS(get(getRouter(state), 'action'))
 
   // It only makes sense to recalculate the `matchPath` whenever the pathname
   // of the location changes. That's why `createMatchSelector` memoizes

src/structure/immutable/get.js

@@ -0,0 +1,9 @@
+import { Iterable } from 'immutable'
+import plainGet from '../plain/get'
+
+const getIn = (state, key) =>
+  Iterable.isIterable(state)
+    ? state.get(key)
+    : plainGet(state, key)
+
+export default getIn

src/structure/immutable/index.js

@@ -1,10 +1,12 @@
 import { Iterable, fromJS } from 'immutable'
 import getIn from './getIn'
+import get from './get'
 
 const structure = {
   fromJS: jsValue => fromJS(jsValue, (key, value) =>
     Iterable.isIndexed(value) ? value.toList() : value.toMap()),
   getIn,
+  get,
   merge: (state, payload) => state.merge(payload),
   toJS: value => Iterable.isIterable(value) ? value.toJS() : value,
 }

src/structure/plain/get.js

@@ -0,0 +1,5 @@
+const get = (state, key) => {
+  return state[key]
+}
+
+export default get

src/structure/plain/index.js

@@ -1,8 +1,10 @@
 import getIn from './getIn'
+import get from './get'
 
 const structure = {
   fromJS: value => value,
   getIn,
+  get,
   merge: (state, payload) => ({ ...state, ...payload }),
   toJS: value => value,
 }

src/structure/seamless-immutable/index.js

@@ -1,11 +1,13 @@
 import SeamlessImmutable from 'seamless-immutable'
 import getIn from '../plain/getIn'
+import get from '../plain/get'
 
 const { static: Immutable } = SeamlessImmutable
 
 const structure = {
   fromJS: value => Immutable.from(value),
   getIn,
+  get,
   merge: (state, payload) => Immutable.merge(state, payload),
   toJS: value => Immutable.asMutable(value)
 }

test/selectors.test.js

@@ -23,6 +23,21 @@ describe("selectors", () => {
     store = createStore(reducer)
   })
 
+  describe("when router not found", () => {
+    beforeEach(() => {
+      const reducer = combineReducers({
+        notTheRouter: connectRouter(history)
+      })
+      store = createStore(reducer)
+    })
+
+    it("throws helpful error", () => {
+      store.dispatch(push('/'))
+      const state = store.getState()
+      expect(() => getLocation(state)).toThrowError('Could not find router reducer in state tree. Are you sure it is mounted under "router"?')
+    })
+  })
+
   describe("getLocation", () => {
     it("gets the location from the state", () => {
       const location = { pathname: "/", hash: '', search: '' }