diff --git a/src/createStore.js b/src/createStore.js
index 0c59bb0689..716710be80 100644
--- a/src/createStore.js
+++ b/src/createStore.js
@@ -1,64 +1,77 @@
 import isPlainObject from 'lodash/isPlainObject'
 import $$observable from 'symbol-observable'
 
-/**
- * These are private action types reserved by Redux.
- * For any unknown actions, you must return the current state.
- * If the current state is undefined, you must return the initial state.
- * Do not reference these action types directly in your code.
- */
 export var ActionTypes = {
   INIT: '@@redux/INIT'
 }
 
-/**
- * Creates a Redux store that holds the state tree.
- * The only way to change the data in the store is to call `dispatch()` on it.
- *
- * There should only be a single store in your app. To specify how different
- * parts of the state tree respond to actions, you may combine several reducers
- * into a single reducer function by using `combineReducers`.
- *
- * @param {Function} reducer A function that returns the next state tree, given
- * the current state tree and the action to handle.
- *
- * @param {any} [initialState] The initial state. You may optionally specify it
- * to hydrate the state from the server in universal apps, or to restore a
- * previously serialized user session.
- * If you use `combineReducers` to produce the root reducer function, this must be
- * an object with the same shape as `combineReducers` keys.
- *
- * @param {Function} enhancer The store enhancer. You may optionally specify it
- * to enhance the store with third-party capabilities such as middleware,
- * time travel, persistence, etc. The only store enhancer that ships with Redux
- * is `applyMiddleware()`.
- *
- * @returns {Store} A Redux store that lets you read the state, dispatch actions
- * and subscribe to changes.
- */
-export default function createStore(reducer, initialState, enhancer) {
-  if (typeof initialState === 'function' && typeof enhancer === 'undefined') {
-    enhancer = initialState
-    initialState = undefined
+function createStoreBase(reducer, initialState, onChange) {
+  var currentState = initialState
+  var isDispatching = false
+
+  function getState() {
+    return currentState
   }
 
-  if (typeof enhancer !== 'undefined') {
-    if (typeof enhancer !== 'function') {
-      throw new Error('Expected the enhancer to be a function.')
+  function dispatch(action) {
+    if (!isPlainObject(action)) {
+      throw new Error(
+        'Actions must be plain objects. ' +
+        'Use custom middleware for async actions.'
+      )
+    }
+    if (typeof action.type === 'undefined') {
+      throw new Error(
+        'Actions may not have an undefined "type" property. ' +
+        'Have you misspelled a constant?'
+      )
+    }
+    if (isDispatching) {
+      throw new Error('Reducers may not dispatch actions.')
+    }
+
+    try {
+      isDispatching = true
+      currentState = reducer(currentState, action)
+    } finally {
+      isDispatching = false
     }
 
-    return enhancer(createStore)(reducer, initialState)
+    onChange()
+    return action
   }
 
+  return {
+    dispatch,
+    getState
+  }
+}
+
+export default function createStore(reducer, initialState, enhancer) {
   if (typeof reducer !== 'function') {
     throw new Error('Expected the reducer to be a function.')
   }
+  if (typeof initialState === 'function' && typeof enhancer === 'undefined') {
+    enhancer = initialState
+    initialState = undefined
+  }
+  if (typeof enhancer !== 'undefined' && typeof enhancer !== 'function') {
+    throw new Error('Expected the enhancer to be a function.')
+  }
 
-  var currentReducer = reducer
-  var currentState = initialState
+  enhancer = enhancer || (x => x)
+  var createFinalStoreBase = enhancer(createStoreBase)
+
+  var storeBase
   var currentListeners = []
   var nextListeners = currentListeners
-  var isDispatching = false
+
+  function onChange() {
+    var listeners = currentListeners = nextListeners
+    for (var i = 0; i < listeners.length; i++) {
+      listeners[i]()
+    }
+  }
 
   function ensureCanMutateNextListeners() {
     if (nextListeners === currentListeners) {
@@ -66,45 +79,12 @@ export default function createStore(reducer, initialState, enhancer) {
     }
   }
 
-  /**
-   * Reads the state tree managed by the store.
-   *
-   * @returns {any} The current state tree of your application.
-   */
-  function getState() {
-    return currentState
-  }
-
-  /**
-   * Adds a change listener. It will be called any time an action is dispatched,
-   * and some part of the state tree may potentially have changed. You may then
-   * call `getState()` to read the current state tree inside the callback.
-   *
-   * You may call `dispatch()` from a change listener, with the following
-   * caveats:
-   *
-   * 1. The subscriptions are snapshotted just before every `dispatch()` call.
-   * If you subscribe or unsubscribe while the listeners are being invoked, this
-   * will not have any effect on the `dispatch()` that is currently in progress.
-   * However, the next `dispatch()` call, whether nested or not, will use a more
-   * recent snapshot of the subscription list.
-   *
-   * 2. The listener should not expect to see all state changes, as the state
-   * might have been updated multiple times during a nested `dispatch()` before
-   * the listener is called. It is, however, guaranteed that all subscribers
-   * registered before the `dispatch()` started will be called with the latest
-   * state by the time it exits.
-   *
-   * @param {Function} listener A callback to be invoked on every dispatch.
-   * @returns {Function} A function to remove this change listener.
-   */
   function subscribe(listener) {
     if (typeof listener !== 'function') {
       throw new Error('Expected listener to be a function.')
     }
 
     var isSubscribed = true
-
     ensureCanMutateNextListeners()
     nextListeners.push(listener)
 
@@ -114,121 +94,43 @@ export default function createStore(reducer, initialState, enhancer) {
       }
 
       isSubscribed = false
-
       ensureCanMutateNextListeners()
       var index = nextListeners.indexOf(listener)
       nextListeners.splice(index, 1)
     }
   }
 
-  /**
-   * Dispatches an action. It is the only way to trigger a state change.
-   *
-   * The `reducer` function, used to create the store, will be called with the
-   * current state tree and the given `action`. Its return value will
-   * be considered the **next** state of the tree, and the change listeners
-   * will be notified.
-   *
-   * The base implementation only supports plain object actions. If you want to
-   * dispatch a Promise, an Observable, a thunk, or something else, you need to
-   * wrap your store creating function into the corresponding middleware. For
-   * example, see the documentation for the `redux-thunk` package. Even the
-   * middleware will eventually dispatch plain object actions using this method.
-   *
-   * @param {Object} action A plain object representing “what changed”. It is
-   * a good idea to keep actions serializable so you can record and replay user
-   * sessions, or use the time travelling `redux-devtools`. An action must have
-   * a `type` property which may not be `undefined`. It is a good idea to use
-   * string constants for action types.
-   *
-   * @returns {Object} For convenience, the same action object you dispatched.
-   *
-   * Note that, if you use a custom middleware, it may wrap `dispatch()` to
-   * return something else (for example, a Promise you can await).
-   */
   function dispatch(action) {
-    if (!isPlainObject(action)) {
-      throw new Error(
-        'Actions must be plain objects. ' +
-        'Use custom middleware for async actions.'
-      )
-    }
-
-    if (typeof action.type === 'undefined') {
-      throw new Error(
-        'Actions may not have an undefined "type" property. ' +
-        'Have you misspelled a constant?'
-      )
-    }
-
-    if (isDispatching) {
-      throw new Error('Reducers may not dispatch actions.')
-    }
-
-    try {
-      isDispatching = true
-      currentState = currentReducer(currentState, action)
-    } finally {
-      isDispatching = false
-    }
-
-    var listeners = currentListeners = nextListeners
-    for (var i = 0; i < listeners.length; i++) {
-      listeners[i]()
-    }
+    return storeBase.dispatch(action)
+  }
 
-    return action
+  function getState() {
+    return storeBase.getState()
   }
 
-  /**
-   * Replaces the reducer currently used by the store to calculate the state.
-   *
-   * You might need this if your app implements code splitting and you want to
-   * load some of the reducers dynamically. You might also need this if you
-   * implement a hot reloading mechanism for Redux.
-   *
-   * @param {Function} nextReducer The reducer for the store to use instead.
-   * @returns {void}
-   */
   function replaceReducer(nextReducer) {
     if (typeof nextReducer !== 'function') {
       throw new Error('Expected the nextReducer to be a function.')
     }
 
-    currentReducer = nextReducer
+    var nextInitialState = storeBase ? getState() : initialState
+    storeBase = createFinalStoreBase(nextReducer, nextInitialState, onChange)
     dispatch({ type: ActionTypes.INIT })
   }
 
-  /**
-   * Interoperability point for observable/reactive libraries.
-   * @returns {observable} A minimal observable of state changes.
-   * For more information, see the observable proposal:
-   * https://github.com/zenparsing/es-observable
-   */
   function observable() {
-    var outerSubscribe = subscribe
     return {
-      /**
-       * The minimal observable subscription method.
-       * @param {Object} observer Any object that can be used as an observer.
-       * The observer object should have a `next` method.
-       * @returns {subscription} An object with an `unsubscribe` method that can
-       * be used to unsubscribe the observable from the store, and prevent further
-       * emission of values from the observable.
-       */
       subscribe(observer) {
         if (typeof observer !== 'object') {
           throw new TypeError('Expected the observer to be an object.')
         }
-
         function observeState() {
           if (observer.next) {
             observer.next(getState())
           }
         }
-
         observeState()
-        var unsubscribe = outerSubscribe(observeState)
+        var unsubscribe = subscribe(observeState)
         return { unsubscribe }
       },
 
@@ -238,15 +140,12 @@ export default function createStore(reducer, initialState, enhancer) {
     }
   }
 
-  // When a store is created, an "INIT" action is dispatched so that every
-  // reducer returns their initial state. This effectively populates
-  // the initial state tree.
-  dispatch({ type: ActionTypes.INIT })
+  replaceReducer(reducer)
 
   return {
     dispatch,
-    subscribe,
     getState,
+    subscribe,
     replaceReducer,
     [$$observable]: observable
   }
diff --git a/test/applyMiddleware.spec.js b/test/applyMiddleware.spec.js
index cb12ab8514..988981a1ce 100644
--- a/test/applyMiddleware.spec.js
+++ b/test/applyMiddleware.spec.js
@@ -94,22 +94,4 @@ describe('applyMiddleware', () => {
       done()
     })
   })
-
-  it('keeps unwrapped dispatch available while middleware is initializing', () => {
-    // This is documenting the existing behavior in Redux 3.x.
-    // We plan to forbid this in Redux 4.x.
-
-    function earlyDispatch({ dispatch }) {
-      dispatch(addTodo('Hello'))
-      return () => action => action
-    }
-
-    const store = createStore(reducers.todos, applyMiddleware(earlyDispatch))
-    expect(store.getState()).toEqual([
-      {
-        id: 1,
-        text: 'Hello'
-      }
-    ])
-  })
 })
diff --git a/test/createStore.spec.js b/test/createStore.spec.js
index 369bb98af3..ec834b2afd 100644
--- a/test/createStore.spec.js
+++ b/test/createStore.spec.js
@@ -495,22 +495,27 @@ describe('createStore', () => {
   })
 
   it('accepts enhancer as the third argument', () => {
+    const spyActions = []
     const emptyArray = []
     const spyEnhancer = vanillaCreateStore => (...args) => {
       expect(args[0]).toBe(reducers.todos)
       expect(args[1]).toBe(emptyArray)
-      expect(args.length).toBe(2)
+      expect(args[2]).toBeA('function')
+      expect(args.length).toBe(3)
       const vanillaStore = vanillaCreateStore(...args)
       return {
         ...vanillaStore,
-        dispatch: expect.createSpy(vanillaStore.dispatch).andCallThrough()
+        dispatch(action) {
+          spyActions.push(action)
+          return vanillaStore.dispatch(action)
+        }
       }
     }
 
     const store = createStore(reducers.todos, emptyArray, spyEnhancer)
     const action = addTodo('Hello')
     store.dispatch(action)
-    expect(store.dispatch).toHaveBeenCalledWith(action)
+    expect(spyActions).toContain(action)
     expect(store.getState()).toEqual([
       {
         id: 1,
@@ -520,21 +525,26 @@ describe('createStore', () => {
   })
 
   it('accepts enhancer as the second argument if initial state is missing', () => {
+    const spyActions = []
     const spyEnhancer = vanillaCreateStore => (...args) => {
       expect(args[0]).toBe(reducers.todos)
       expect(args[1]).toBe(undefined)
-      expect(args.length).toBe(2)
+      expect(args[2]).toBeA('function')
+      expect(args.length).toBe(3)
       const vanillaStore = vanillaCreateStore(...args)
       return {
         ...vanillaStore,
-        dispatch: expect.createSpy(vanillaStore.dispatch).andCallThrough()
+        dispatch(action) {
+          spyActions.push(action)
+          return vanillaStore.dispatch(action)
+        }
       }
     }
 
     const store = createStore(reducers.todos, spyEnhancer)
     const action = addTodo('Hello')
     store.dispatch(action)
-    expect(store.dispatch).toHaveBeenCalledWith(action)
+    expect(spyActions).toContain(action)
     expect(store.getState()).toEqual([
       {
         id: 1,