(test): add initial test harness

- add 1 simple test that just runs the persist function and ensures
  it doesn't persist anything if an action hasn't been used

- change test script to use jest, and move prev test to test:pub
  - (ci): change CI to run test and test:pub

- (deps): add @types/jest to devDeps
  - also add reference in spec so that the types are loaded properly
    - not sure why the reference is needed, but it would error without
      - and plain `import`ing jest or @types/jest would also error

.travis.yml

@@ -1,3 +1,6 @@
 language: node_js
 # default is the very old 0.10.48; match local version instead
 node_js: '8.9'
+
+script: npm test
+after_script: npm run test:pub

package.json

@@ -29,7 +29,8 @@
   "scripts": {
     "start": "tsdx watch",
     "build": "tsdx build",
-    "test": "npm run build && npm pack",
+    "test": "tsdx test",
+    "test:pub": "npm run build && npm pack",
     "pub": "npm run build && npm publish",
     "changelog": "changelog-maker"
   },
@@ -39,6 +40,7 @@
   "dependencies": {},
   "devDependencies": {
     "@agilgur5/changelog-maker": "^3.0.0",
+    "@types/jest": "^24.0.23",
     "mobx": "^5.11.0",
     "mobx-state-tree": "^3.14.0",
     "tsdx": "^0.7.2",

test/fixtures.ts

@@ -0,0 +1,6 @@
+import { types } from 'mobx-state-tree'
+
+export const UserStore = types.model('UserStore', {
+  name: 'John Doe',
+  age: 32
+})

test/index.spec.ts

@@ -0,0 +1,15 @@
+/// <reference types="@types/jest" />
+
+import { persist } from '../src/index'
+import { UserStore } from './fixtures'
+
+describe('initialization', () => {
+  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)
+  })
+})