Unit Testing of Cloud Functions

This page describes best practices and tools for writing unit tests for your functions, such as tests that would be a part of a Continuous Integration (CI) system. To make testing easier, Firebase provides the Firebase Test SDK for Cloud Functions. It is distributed on npm as firebase-functions-test, and is a companion test SDK to firebase-functions. The Firebase Test SDK for Cloud Functions:

  • Takes care of the appropriate setup and teardown for your tests, such as setting and unsetting environment variables needed by firebase-functions.
  • Generates sample data and event context, so that you only have to specify the fields that are relevant to your test.

Test setup

Install both firebase-functions-test and Mocha, a testing framework, by running the following commands in your functions folder:

npm install --save-dev firebase-functions-test
npm install --save-dev mocha

Next create a test folder inside the functions folder, create a new file inside it for your test code, and name it something like index.test.js.

Finally, modify functions/package.json to add the following:

"scripts": {
  "test": "mocha --reporter spec"

Once you have written the tests, you can run them by running npm test inside your functions directory.

Initializing Firebase Test SDK for Cloud Functions

There are two ways to use firebase-functions-test:

  1. Offline mode: Write siloed and offline unit tests with no side effects. This means that any method calls that interact with a Firebase product (e.g. writing to the database or creating a user) needs to be stubbed.
  2. Online mode: Write tests that interact with a Firebase project dedicated to testing so that database writes, user creates, etc. would actually happen, and your test code can inspect the results. This also means that other Google SDKs used in your functions will work as well.

Initialize SDK in offline mode

If you would like to write completely offline tests, you can initialize the SDK without any parameters:

// At the top of test/index.test.js
const test = require('firebase-functions-test')();

Initialize SDK in online mode

If you would like to write tests that interact with a test project, you need to supply the project config values that are needed for initializing the app through firebase-admin, and the path to a service account key file.

To get your Firebase project's config values:

  1. Go to Firebase Console,
  2. Select your project, then click Add Firebase to your web app, config values are displayed in the pop up.

To create a key file:

  1. Open the Service Accounts pane of the Google Cloud Console.
  2. Select the App Engine default service account, and use the options menu at the right to select Create key.
  3. When prompted, select JSON for the key type, and click Create.

After saving the key file, initialize the SDK as follows:

// At the top of test/index.test.js
const test = require('firebase-functions-test')({
  databaseURL: 'https://my-project.firebaseio.com',
  storageBucket: 'my-project.appspot.com',
  projectId: 'my-project',
}, 'path/to/serviceAccountKey.json');

Mocking config values

If you use functions.config() in your functions code, you can mock the config values. For example, if functions/index.js contains the following code:

const functions = require('firebase-functions');
const key = functions.config().stripe.key;

Then you can mock the value inside your test file like so:

// Mock functions config values
test.mockConfig({ stripe: { key: '23wr42ewr34' }});

Importing your functions

To import your functions, use require to import your main functions file as a module. Be sure to only do this after initializing firebase-functions-test, and mocking config values.

// after firebase-functions-test has been initialized
const myFunctions = require('../index.js'); // relative path to functions code

If you initialized firebase-functions-test in offline mode, and you have admin.initializeApp() in your functions code, then you need to stub it before importing your functions:

// If index.js calls admin.initializeApp at the top of the file,
// we need to stub it out before requiring index.js. This is because the
// functions will be executed as a part of the require process.
// Here we stub admin.initializeApp to be a dummy function that doesn't do anything.
adminInitStub = sinon.stub(admin, 'initializeApp');
// Now we can require index.js and save the exports inside a namespace called myFunctions.
myFunctions = require('../index');

Testing background (non-HTTP) functions

The process for testing non-HTTP functions involves the following steps:

  1. Wrap the function you would like to test with the test.wrap method
  2. Construct test data
  3. Invoke the wrapped function with the test data you constructed and any event context fields you'd like to specify.
  4. Make assertions about behavior.

First wrap the function you'd like to test. Let's say you have a function in functions/index.js called makeUppercase, which you'd like to test. Write the following in functions/test/index.test.js

// "Wrap" the makeUpperCase function from index.js
const myFunctions = require('../index.js');
const wrapped = test.wrap(myFunctions.makeUppercase);

wrapped is a function which invokes makeUppercase when it is called. wrapped takes 2 parameters:

  1. data (required): the data to send to makeUppercase. This directly corresponds to the first parameter sent to the function handler that you wrote. firebase-functions-test provides methods for constructing custom data or example data.
  2. eventContextOptions (optional): fields of the event context that you'd like to specify. The event context is the second parameter sent to the function handler that you wrote. If you do not include an eventContextOptions parameter when calling wrapped, an event context is still generated with sensible fields. You can override some of the generated fields by specifying them here. Note that you only have to include the fields that you'd like to override. Any fields that you did not override are generated.
const data = … // See next section for constructing test data

// Invoke the wrapped function without specifying the event context.

// Invoke the function, and specify params
wrapped(data, {
  params: {
    pushId: '234234'

// Invoke the function, and specify auth and auth Type (for real time database functions only)
wrapped(data, {
  auth: {
    uid: 'jckS2Q0'
  authType: 'USER'

// Invoke the function, and specify all the fields that can be specified
wrapped(data, {
  eventId: 'abc',
  timestamp: '2018-03-23T17:27:17.099Z',
  params: {
    pushId: '234234'
  auth: {
    uid: 'jckS2Q0' // only for real time database functions
  authType: 'USER' // only for real time database functions

Constructing test data

The first parameter of a wrapped function is the test data to invoke the underlying function with. There are a number of ways to construct test data.

Using custom data

firebase-functions-test has a number of functions for constructing data needed to test your functions. For example, use test.firestore.makeDocumentSnapshot to create a Firestore DocumentSnapshot. The first argument is the data, and the second argument is the full reference path, and there is an optional third argument for other properties of the snapshot you can specify.

// Make snapshot
const snap = test.firestore.makeDocumentSnapshot({foo: 'bar'}, 'document/path');
// Call wrapped function with the snapshot
const wrapped = test.wrap(myFunctions.myFirestoreDeleteFunction);

If you are testing an onUpdate or onWrite function, you'll need to create two snapshots: one for the before state and one for the after state. Then, you can use the makeChange method to create a Change object with these snapshots.

// Make snapshot for state of database beforehand
const beforeSnap = test.firestore.makeDocumentSnapshot({foo: 'bar'}, 'document/path');
// Make snapshot for state of database after the change
const afterSnap = test.firestore.makeDocumentSnapshot({foo: 'faz'}, 'document/path');
const change = test.makeChange(beforeSnap, afterSnap);
// Call wrapped function with the Change object
const wrapped = test.wrap(myFunctions.myFirestoreUpdateFunction);

See the API reference for similar functions for all the other data types.

Using example data

If you don't need to customize the data used in the your tests, then firebase-functions-test offers methods for generating example data for each function type.

// For Firestore onCreate or onDelete functions
const snap = test.firestore.exampleDocumentSnapshot();
// For Firestore onUpdate or onWrite functions
const change = test.firestore.exampleDocumentSnapshotChange();

See the API reference for methods for getting example data for every function type.

Using stubbed data (for offline mode)

If you initialized the SDK in offline mode, and are testing a Firestore or Database function, you may find it easier to use a plain object with stubs instead of creating an actual DocumentSnapshot or DataSnapshot and then stubbing its methods.

Let's say you are writing a unit test for the following function:

exports.makeUppercase = functions.database.ref('/messages/{pushId}/original')
    .onCreate((snap, context) => {
// [END makeUppercaseTrigger]
      // [START makeUppercaseBody]
      // Grab the current value of what was written to the Realtime Database.
      const original = snap.val();
      console.log('Uppercasing', context.params.pushId, original);
      const uppercase = original.toUpperCase();
      // You must return a Promise when performing asynchronous tasks inside a Functions such as
      // writing to the Firebase Realtime Database.
      // Setting an "uppercase" sibling in the Realtime Database returns a Promise.
      return snap.ref.parent.child('uppercase').set(uppercase);
      // [END makeUppercaseBody]

Inside of the function, snap is used twice:

  • snap.val()
  • snap.ref.parent.child('uppercase').set(uppercase)

In test code, create a plain object where both of these code paths will work, and use Sinon to stub the methods.

// The following lines creates a fake snapshot, 'snap', which returns 'input' when snap.val() is called,
// and returns true when snap.ref.parent.child('uppercase').set('INPUT') is called.
const snap = {
  val: () => 'input',
  ref: {
    parent: {
      child: childStub,
childStub.withArgs(childParam).returns({ set: setStub });

Making assertions

After initializing the SDK, wrapping the functions, and constructing data, you can invoke the wrapped functions with the constructed data and make assertions about behavior. You can use a library such as Chai for making these assertions.

Making assertions in offline mode

You can make assertions about the expected return value of the function:

const childParam = 'uppercase';
const setParam = 'INPUT';
// Stubs are objects that fake and/or record function calls.
// These are excellent for verifying that functions have been called and to validate the
// parameters passed to those functions.
const childStub = sinon.stub();
const setStub = sinon.stub();
// The following lines creates a fake snapshot, 'snap', which returns 'input' when snap.val() is called,
// and returns true when snap.ref.parent.child('uppercase').set('INPUT') is called.
const snap = {
  val: () => 'input',
  ref: {
    parent: {
      child: childStub,
childStub.withArgs(childParam).returns({ set: setStub });
// Wrap the makeUppercase function.
const wrapped = test.wrap(myFunctions.makeUppercase);
// Since we've stubbed snap.ref.parent.child(childParam).set(setParam) to return true if it was
// called with the parameters we expect, we assert that it indeed returned true.
return assert.equal(wrapped(snap), true);

You can also used Sinon spies to assert that certain methods have been called, and with parameters you expect.

Making assertions in online mode

If you initialized the Firebase Test SDK for Cloud Functions in online mode, you can assert that the desired actions (such as a database write) has taken place by using the firebase-admin SDK.

The example below asserts that 'INPUT' has been written into the real time database of the test project.

// 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');

Testing HTTP functions

If you are testing HTTP onRequest functions, you should use firebase-functions-test if:

  • You use functions.config()
  • Your function interacts with a Firebase project or other Google APIs, and you'd like to use a real Firebase project and its credentials for your tests.

An HTTP onRequest function takes two parameters: a request object and a response object. Here is how you might test the addMessage() example function:

  • Override the redirect function in the response object, since sendMessage() calls it.
  • Within the redirect function, use chai.assert to help make assertions about what parameters the redirect function should be called with:
// A fake request object, with req.query.text set to 'input'
const req = { query: {text: 'input'} };
// A fake response object, with a stubbed redirect function which asserts that it is called
// with parameters 303, 'new_ref'.
const res = {
  redirect: (code, url) => {
    assert.equal(code, 303);
    assert.equal(url, 'new_ref');

// Invoke addMessage with our fake request and response objects. This will cause the
// assertions in the response object to be evaluated.
myFunctions.addMessage(req, res);

Test cleanup

At the very end of your test code, call the cleanup function. This unsets environment variables that the SDK set when it was initialized, and deletes Firebase apps that may have been created if you used the SDK to create a real time database DataSnapshot or Firestore DocumentSnapshot.


Review complete examples and learn more

You can review the complete examples on the Firebase GitHub repository.

To learn more, refer to the API reference for firebase-functions-test.

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