diff --git a/package.json b/package.json
index ef5ac9f665..3f59918dd7 100644
--- a/package.json
+++ b/package.json
@@ -106,7 +106,7 @@
     ]
   },
   "jest": {
-    "testRegex": "(/test/.*\\.spec\\.js)$"
+    "testRegex": "(/test/.*\\.spec\\.[tj]s)$"
   },
   "sideEffects": false
 }
diff --git a/test/createStore.spec.js b/test/createStore.spec.ts
similarity index 90%
rename from test/createStore.spec.js
rename to test/createStore.spec.ts
index f3700e2d6d..5e8ef1db50 100644
--- a/test/createStore.spec.js
+++ b/test/createStore.spec.ts
@@ -1,4 +1,11 @@
-import { createStore, combineReducers } from '../'
+import {
+  createStore,
+  combineReducers,
+  Reducer,
+  StoreEnhancer,
+  Action,
+  Store
+} from '..'
 import {
   addTodo,
   dispatchInMiddle,
@@ -9,7 +16,7 @@ import {
   unknownAction
 } from './helpers/actionCreators'
 import * as reducers from './helpers/reducers'
-import { from } from 'rxjs'
+import { from, ObservableInput } from 'rxjs'
 import { map } from 'rxjs/operators'
 import $$observable from 'symbol-observable'
 
@@ -26,11 +33,11 @@ describe('createStore', () => {
   })
 
   it('throws if reducer is not a function', () => {
-    expect(() => createStore()).toThrow()
+    expect(() => createStore(undefined)).toThrow()
 
-    expect(() => createStore('test')).toThrow()
+    expect(() => createStore(('test' as unknown) as Reducer)).toThrow()
 
-    expect(() => createStore({})).toThrow()
+    expect(() => createStore(({} as unknown) as Reducer)).toThrow()
 
     expect(() => createStore(() => {})).not.toThrow()
   })
@@ -568,13 +575,23 @@ describe('createStore', () => {
   })
 
   it('throws if enhancer is neither undefined nor a function', () => {
-    expect(() => createStore(reducers.todos, undefined, {})).toThrow()
+    expect(() =>
+      createStore(reducers.todos, undefined, ({} as unknown) as StoreEnhancer)
+    ).toThrow()
 
-    expect(() => createStore(reducers.todos, undefined, [])).toThrow()
+    expect(() =>
+      createStore(reducers.todos, undefined, ([] as unknown) as StoreEnhancer)
+    ).toThrow()
 
     expect(() => createStore(reducers.todos, undefined, null)).toThrow()
 
-    expect(() => createStore(reducers.todos, undefined, false)).toThrow()
+    expect(() =>
+      createStore(
+        reducers.todos,
+        undefined,
+        (false as unknown) as StoreEnhancer
+      )
+    ).toThrow()
 
     expect(() =>
       createStore(reducers.todos, undefined, undefined)
@@ -586,25 +603,27 @@ describe('createStore', () => {
 
     expect(() => createStore(reducers.todos, [])).not.toThrow()
 
-    expect(() => createStore(reducers.todos, {})).not.toThrow()
+    expect(() =>
+      createStore<{}, Action, {}, {}>(reducers.todos, {})
+    ).not.toThrow()
   })
 
   it('throws if nextReducer is not a function', () => {
     const store = createStore(reducers.todos)
 
-    expect(() => store.replaceReducer()).toThrow(
+    expect(() => store.replaceReducer(undefined)).toThrow(
       'Expected the nextReducer to be a function.'
     )
 
-    expect(() => store.replaceReducer(() => {})).not.toThrow()
+    expect(() => store.replaceReducer(() => [])).not.toThrow()
   })
 
   it('throws if listener is not a function', () => {
     const store = createStore(reducers.todos)
 
-    expect(() => store.subscribe()).toThrow()
+    expect(() => store.subscribe(undefined)).toThrow()
 
-    expect(() => store.subscribe('')).toThrow()
+    expect(() => store.subscribe(('' as unknown) as () => void)).toThrow()
 
     expect(() => store.subscribe(null)).toThrow()
 
@@ -654,11 +673,11 @@ describe('createStore', () => {
     })
 
     it('should pass an integration test with no unsubscribe', () => {
-      function foo(state = 0, action) {
+      function foo(state = 0, action: Action) {
         return action.type === 'foo' ? 1 : state
       }
 
-      function bar(state = 0, action) {
+      function bar(state = 0, action: Action) {
         return action.type === 'bar' ? 2 : state
       }
 
@@ -667,7 +686,7 @@ describe('createStore', () => {
       const results = []
 
       observable.subscribe({
-        next(state) {
+        next(state: any) {
           results.push(state)
         }
       })
@@ -683,11 +702,11 @@ describe('createStore', () => {
     })
 
     it('should pass an integration test with an unsubscribe', () => {
-      function foo(state = 0, action) {
+      function foo(state = 0, action: Action) {
         return action.type === 'foo' ? 1 : state
       }
 
-      function bar(state = 0, action) {
+      function bar(state = 0, action: Action) {
         return action.type === 'bar' ? 2 : state
       }
 
@@ -696,7 +715,7 @@ describe('createStore', () => {
       const results = []
 
       const sub = observable.subscribe({
-        next(state) {
+        next(state: any) {
           results.push(state)
         }
       })
@@ -709,25 +728,26 @@ describe('createStore', () => {
     })
 
     it('should pass an integration test with a common library (RxJS)', () => {
-      function foo(state = 0, action) {
+      function foo(state = 0, action: Action) {
         return action.type === 'foo' ? 1 : state
       }
 
-      function bar(state = 0, action) {
+      function bar(state = 0, action: Action) {
         return action.type === 'bar' ? 2 : state
       }
 
-      const store = createStore(combineReducers({ foo, bar }))
+      const store: ObservableInput<{ foo: number; bar: number }> = createStore(
+        combineReducers({ foo, bar })
+      )
       const observable = from(store)
       const results = []
 
       const sub = observable
         .pipe(map(state => ({ fromRx: true, ...state })))
         .subscribe(state => results.push(state))
-
-      store.dispatch({ type: 'foo' })
+      ;(store as Store).dispatch({ type: 'foo' })
       sub.unsubscribe()
-      store.dispatch({ type: 'bar' })
+      ;(store as Store).dispatch({ type: 'bar' })
 
       expect(results).toEqual([
         { foo: 0, bar: 0, fromRx: true },
@@ -758,7 +778,7 @@ describe('createStore', () => {
       })
     )
 
-    expect(console.error.mock.calls.length).toBe(0)
+    expect((console.error as any).mock.calls.length).toBe(0)
     console.error = originalConsoleError
   })
 
@@ -766,15 +786,22 @@ describe('createStore', () => {
     const rootReducer = combineReducers(reducers)
     const dummyEnhancer = f => f
     expect(() =>
-      createStore(rootReducer, dummyEnhancer, dummyEnhancer)
+      // use a fake pre-loaded state to get a valid createStore signature
+      createStore(rootReducer, (dummyEnhancer as unknown) as {}, dummyEnhancer)
     ).toThrow()
   })
 
   it('throws if passing several enhancer functions with preloaded state', () => {
     const rootReducer = combineReducers(reducers)
-    const dummyEnhancer = f => f
+    const dummyEnhancer = (f: any) => f
     expect(() =>
-      createStore(rootReducer, { todos: [] }, dummyEnhancer, dummyEnhancer)
+      // work around the type so we can call this poorly
+      (createStore as any)(
+        rootReducer,
+        { todos: [] },
+        dummyEnhancer,
+        dummyEnhancer
+      )
     ).toThrow()
   })
 })