Fix db.test.

- Fake timers cause an infinite loop on _.debounce.
- Jest v26 contains a 'modern' option for useFakeTimers, but create-react-app uses an older version of jest
jestjs/jest#3465 (comment)
- Mock debounce
- Test fails without a dummy call to the database. Why?

src/action-creators/loadLocalState.js

@@ -7,15 +7,12 @@ import { never } from '../util'
 /** Loads the local state from the IndexedDB database. */
 const loadLocalState = () => async (dispatch, getState) => {
 
-  // TODO: Fix IndexedDB during tests
-  const test = process.env.NODE_ENV === 'test'
-
   // load helpers and settings from local database
   const [{
     cursor: localUrl,
     lastUpdated,
     recentlyEdited,
-  }, settings] = test ? [{}] : await Promise.all([
+  }, settings] = await Promise.all([
     db.getHelpers(),
     db.getContext([EM_TOKEN, 'Settings'])
   ])

src/db.test.js

@@ -1,23 +1,72 @@
 import { store } from './store'
 import * as db from './data-providers/dexie'
 import { initialize } from './initialize'
-import { hashThought } from './util'
 import { getThought } from './selectors'
 
+// mock debounce to use 0 delay
+jest.mock('lodash', () => ({
+  ...jest.requireActual('lodash'),
+
+  // jest.mock must be inline
+  // possible workarounds: https://stackoverflow.com/questions/40465047/how-can-i-mock-an-es6-module-import-using-jest
+  debounce: jest.fn().mockImplementation((callback, delay) => {
+    let timer = null
+    let pendingArgs = null
+
+    const cancel = jest.fn(() => {
+      if (timer) {
+        clearTimeout(timer)
+      }
+      timer = null
+      pendingArgs = null
+    })
+
+    const flush = jest.fn(() => {
+      if (timer) {
+        callback(...pendingArgs)
+        cancel()
+      }
+    })
+
+    const wrapped = (...args) => {
+      cancel()
+
+      pendingArgs = args
+
+      // TODO: why doesn't jest.runAllTimers work here?
+      // use 0 instead of given delay as a workaround
+      timer = setTimeout(flush, 0)
+    }
+
+    wrapped.cancel = cancel
+    wrapped.flush = flush
+    wrapped.delay = delay
+
+    return wrapped
+  }),
+}))
+
+
 beforeAll(async () => {
   await initialize()
-})
 
-// TODO: Make pass with iterative loading
-it.skip('load settings into indexedDB on initialization', async () => {
-  const hash = hashThought('Settings')
+  // fake timers cause an infinite loop on _.debounce
+  // Jest v26 contains a 'modern' option for useFakeTimers, but create-react-app uses an older version of jest
+  // https://github.com/facebook/jest/issues/3465#issuecomment-504908570
+  jest.useFakeTimers()
+  jest.runAllTimers()
+})
 
+it('load settings into indexedDB on initialization', async () => {
   const thoughtState = getThought(store.getState(), 'Settings')
 
   expect(thoughtState).not.toBeUndefined()
   expect(thoughtState.contexts).toHaveLength(1)
 
-  const thoughtDB = await db.getThought(hash)
+  // TODO: Tests fail without a dummy call to the database. Why?
+  await db.getHelpers()
+
+  const thoughtDB = await db.getThought('Settings')
 
   expect(thoughtDB).not.toBeUndefined()
   expect(thoughtDB.contexts).toHaveLength(1)