add a test for readme example

* which currently fails => need to update code..

jest.config.js

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

package.json

@@ -13,6 +13,7 @@
   "scripts": {
     "clean": "rimraf ./lib",
     "build": "yarn clean && tsc",
+    "test": "jest",
     "prepublishOnly": "yarn build"
   },
   "devDependencies": {

src/LiveStorage.node.test.ts

@@ -0,0 +1,11 @@
+import { LiveStorage } from './LiveStorage'
+
+test('REAME example', () => {
+  const scenario = () => {
+    const posts = new LiveStorage('posts', [] as any[])
+    posts.update((prevPosts) => prevPosts.concat({ title: 'Brave new world' }))
+
+    expect(posts.value).toEqual([{ title: 'Brave new world' }])
+  }
+  expect(scenario).not.toThrow()
+})

src/LiveStorage.ts

@@ -5,7 +5,10 @@ interface BroadcastChannelMessageType<ValueType> {
   }
 }
 
-const IS_BROWSER = typeof window !== 'undefined'
+// Jest jsdom contains `window` object in Node => detect required features
+const IS_BROWSER =
+  typeof BroadcastChannel !== 'undefined' &&
+  typeof sessionStorage !== 'undefined'
 
 export class LiveStorage<ValueType> {
   public id: string