(test): ensure snapshots are persisted properly

- add one action in order to trigger onSnapshot
- add tests for no options, whitelist, and blacklist
  - add comment about shallow cloning snapshot being required due to
    non-configurable properties
    - this code was borrowed from a gist, so I didn't actually know
      why it was necessary until I tried running the test without
      shallow cloning

src/index.ts

@@ -34,6 +34,7 @@ export const persist: IArgs = (name, store, options = {}) => {
   const blacklistDict = arrToDict(blacklist)
 
   onSnapshot(store, (_snapshot: StrToAnyMap) => {
+    // need to shallow clone as otherwise properties are non-configurable (https://github.com/agilgur5/mst-persist/pull/21#discussion_r348105595)
     const snapshot = { ..._snapshot }
     Object.keys(snapshot).forEach((key) => {
       if (whitelist && !whitelistDict[key]) {

test/fixtures.ts

@@ -3,4 +3,8 @@ import { types } from 'mobx-state-tree'
 export const UserStore = types.model('UserStore', {
   name: 'John Doe',
   age: 32
-})
+}).actions((self) => ({
+  changeName(name: string) {
+    self.name = name
+  }
+}))

test/index.spec.ts

@@ -1,15 +1,53 @@
 /// <reference types="@types/jest" />
+import { getSnapshot } from 'mobx-state-tree'
 
 import { persist } from '../src/index'
 import { UserStore } from './fixtures'
 
-describe('initialization', () => {
+function getItem(key: string) {
+  const item = window.localStorage.getItem(key)
+  return item ? JSON.parse(item) : null // can only parse strings
+}
+
+describe('basic persist options', () => {
   beforeEach(() => window.localStorage.clear())
 
   it('should persist nothing if no actions are used', async () => {
     const user = UserStore.create()
     await persist('user', user)
 
-    expect(window.localStorage.getItem('user')).toBe(null)
+    expect(getItem('user')).toBe(null)
+  })
+
+  it('should persist snapshot when action used', async () => {
+    const user = UserStore.create()
+    await persist('user', user)
+
+    user.changeName('Joe') // fire action to trigger onSnapshot
+    expect(getItem('user')).toStrictEqual(getSnapshot(user))
+  })
+
+  it('should whitelist', async () => {
+    const user = UserStore.create()
+    await persist('user', user, {
+      whitelist: ['name']
+    })
+
+    user.changeName('Joe') // fire action to trigger onSnapshot
+    const snapshot = { ...getSnapshot(user) } // need to shallow clone as otherwise properties are non-configurable (https://github.com/agilgur5/mst-persist/pull/21#discussion_r348105595)
+    delete snapshot['age']
+    expect(getItem('user')).toStrictEqual(snapshot)
+  })
+
+  it('should blacklist', async () => {
+    const user = UserStore.create()
+    await persist('user', user, {
+      blacklist: ['age']
+    })
+
+    user.changeName('Joe') // fire action to trigger onSnapshot
+    const snapshot = { ...getSnapshot(user) } // need to shallow clone as otherwise properties are non-configurable (https://github.com/agilgur5/mst-persist/pull/21#discussion_r348105595)
+    delete snapshot['age']
+    expect(getItem('user')).toStrictEqual(snapshot)
   })
 })