Documentation for Cloud Functions online unit test is misleading.

Steps to reproduce

Here's the snipper from the documenatation:

  describe('makeUpperCase', () => {
    // Test Case: setting messages/11111/original to 'input' should cause 'INPUT' to be written to
    // messages/11111/uppercase
    it('should upper case input and write it to /uppercase', () => {
      // [START assertOnline]
      // Create a DataSnapshot with the value 'input' and the reference path 'messages/11111/original'.
      const snap = test.database.makeDataSnapshot('input', 'messages/11111/original');

      // Wrap the makeUppercase function
      const wrapped = test.wrap(myFunctions.makeUppercase);
      // Call the wrapped function with the snapshot you constructed.
      return wrapped(snap).then(() => {
        // Read the value of the data at messages/11111/uppercase. Because `admin.initializeApp()` is
        // called in functions/index.js, there's already a Firebase app initialized. Otherwise, add
        // `admin.initializeApp()` before this line.
        return admin.database().ref('messages/11111/uppercase').once('value').then((createdSnap) => {
          // Assert that the value is the uppercased version of our input.
          assert.equal(createdSnap.val(), 'INPUT');
        });
      });
      // [END assertOnline]
    })
  });

Actual behavior

IMO the following part is misleasing:

// Because `admin.initializeApp()` is
// called in functions/index.js, there's already a Firebase app initialized. Otherwise, add
// `admin.initializeApp()` before this line.

Calling admin.initializeApp() without arguments doesn't make much sense when running unit tests locally. When this method is called without arguments it reads the config from FIREBASE_CONFIG. Normally, when runing test on localhost such a variable doesn't exist.

Expected behavior

The documenation should, at least, mention that the funcion must either be called with credentials object passed as an argument, or the FIREBASE_CONFIG env variable must be set before running unit tests.

firebase / firebase-functions-test