(test): ensure transforms apply and do so in correct order

- yay back to 100% code coverage now!
  - there's a transform branch missing bc there's no test case where
    there's something in storage and at least one toStorage-only
    transform
    - but covering this else branch doesn't really matter

- create some transform fixtures
- abstract out a setItem function

- should consider splitting the tests and fixtures into separate files
  soon, esp now that we have a few different test suites here

test/fixtures.ts

@@ -1,5 +1,7 @@
 import { types } from 'mobx-state-tree'
 
+import { ITransform, ITransformArgs } from '../src/index'
+
 export const UserStoreF = types.model('UserStore', {
   name: 'John Doe',
   age: 32
@@ -13,3 +15,19 @@ export const persistedDataF = {
   name: 'Persisted Name',
   age: 35
 }
+
+function changeName (name: string) {
+  const changeNameTransform: ITransformArgs = function (snapshot) {
+    snapshot.name = name
+    return snapshot
+  }
+  return changeNameTransform
+}
+
+export function storeNameAsF (name: string): ITransform {
+  return {toStorage: changeName(name)}
+}
+
+export function retrieveNameAsF (name: string): ITransform {
+  return {fromStorage: changeName(name)}
+}

test/index.spec.ts

@@ -2,13 +2,17 @@
 import { getSnapshot } from 'mobx-state-tree'
 
 import { persist } from '../src/index'
-import { UserStoreF, persistedDataF } from './fixtures'
+import { UserStoreF, persistedDataF, storeNameAsF, retrieveNameAsF } from './fixtures'
 
 function getItem(key: string) {
   const item = window.localStorage.getItem(key)
   return item ? JSON.parse(item) : null // can only parse strings
 }
 
+function setItem(key: string, value: object) {
+  return window.localStorage.setItem(key, JSON.stringify(value))
+}
+
 describe('basic persist functionality', () => {
   beforeEach(() => window.localStorage.clear())
 
@@ -28,15 +32,16 @@ describe('basic persist functionality', () => {
   })
 
   it('should load persisted data', async () => {
-    window.localStorage.setItem('user', JSON.stringify(persistedDataF))
+    setItem('user', persistedDataF)
 
     const user = UserStoreF.create()
     await persist('user', user)
+
     expect(getSnapshot(user)).toStrictEqual(persistedDataF)
   })
 })
 
-describe('persist options', () => {
+describe('basic persist options', () => {
   beforeEach(() => window.localStorage.clear())
 
   it('shouldn\'t jsonify', async () => {
@@ -74,3 +79,30 @@ describe('persist options', () => {
     expect(getItem('user')).toStrictEqual(snapshot)
   })
 })
+
+describe('transforms', () => {
+  beforeEach(() => window.localStorage.clear())
+
+  it('should apply toStorage transforms in order', async () => {
+    const user = UserStoreF.create()
+    await persist('user', user, {
+      transforms: [storeNameAsF('Jack'), storeNameAsF('Joe')]
+    })
+
+    user.changeName('Not Joe') // fire action to trigger onSnapshot
+    expect(getItem('user').name).toBe('Joe')
+  })
+
+  it('should apply fromStorage transforms in reverse order', async () => {
+    const persistedData = {...persistedDataF}
+    persistedData.name = 'Not Joe'
+    setItem('user', persistedData)
+
+    const user = UserStoreF.create()
+    await persist('user', user, {
+      transforms: [retrieveNameAsF('Joe'), retrieveNameAsF('Jack')]
+    })
+
+    expect(getSnapshot(user).name).toBe('Joe')
+  })
+})