added jest and a new test

jest.config.js

@@ -0,0 +1,4 @@
+module.exports = {
+  preset: 'ts-jest',
+  testEnvironment: 'node'
+};

package.json

@@ -5,17 +5,21 @@
   "main": "index.js",
   "scripts": {
     "start": "webpack-dev-server --mode development --progress",
-    "build": "webpack --mode production"
+    "build": "webpack --mode production",
+    "test": "jest --watch"
   },
   "keywords": [],
   "author": "",
   "license": "ISC",
   "devDependencies": {
+    "@types/jest": "^23.3.13",
     "@types/react": "^16.7.20",
     "@types/react-dom": "^16.0.11",
-    "@types/redux": "^3.6.0",
     "@types/react-redux": "^7.0.0",
+    "@types/redux": "^3.6.0",
     "html-webpack-plugin": "^3.2.0",
+    "jest": "^23.6.0",
+    "ts-jest": "^23.10.5",
     "ts-loader": "^5.3.3",
     "typescript": "^3.2.4",
     "webpack": "^4.28.4",

playground/src/reducers/__tests__/reducers.spec.ts

@@ -0,0 +1,10 @@
+import { reducer } from '../index';
+
+describe('reducers', () => {
+  it('should add item to the list', () => {
+    const newStore = reducer({ todos: {}, filter: 'all' }, { type: 'add', label: 'hello' });
+    const keys = Object.keys(newStore.todos);
+    expect(keys.length).toBe(1);
+    expect(newStore.todos[keys[0]].label).toBe('hello');
+  });
+});