If you’ve been writing and deploying Firebase cloud functions for your project, you know it takes a good minute (at least) to deploy changes. When in the development phase, it’s beneficial to be able to test them locally rather than deploying to Firebase with each change to run a test.

On the services I run on Firebase I have very few, simple cloud functions, and I have not required extensive testing so far. But recently I updated Firebase from version 8 to version 10, going from the namespaced API to the modular API while also migrating coud functions from gen1 to gen2 and upgrading all OpenAI functions from v3 to v4 (read more about working with the OpenAI API here), so local testing was a must. Here’s how the Firebase Emulator Suite was set up to be able to test the cloud function that calls the OpenAI endpoint locally before deployment:

1. When inside of your Firebase project, add emulators by running in the terminal:

firebase init emulators

NOTE: this assumes you already have the Firebase CLI installed. If not, install it:

pnpm install -g firebase-tools

After running the command to initiate emulators, you can choose to install the emulators you need. In this case, we only talk about the functions emulator, but there are emulators for Firestore (database), the auth service and more.

Note: you need the Java runtime to run the emulators (they are based on Java). The easiest if you don’t have Java installed is to go to https://adoptium.net/ and download it from there.

2. Add any API keys and other secrets in a .env.local file inside the functions folder of your project. Add .env.local to your gitignore

// .gitignore
# local env files

3. In your firebase config file, add this to connect the emulator if you’re in development mode (this is if you are using Vite):

if (!globalThis.emulation && import.meta.env.MODE === "development") {
  //arguments here depend on your setup. First is the const you assigned “getFunctions(...) to,
  //second is where the functions are served, and third is the port
  connectFunctionsEmulator(functions, "", 5001);

  // set emulation flag to avoid connecting twice
  globalThis.emulation = true;

4. You can now start the emulator:

firebase emulators:start --only functions

5. To make this more convenient, you can add this to your scripts in the package.json file inside your functions folder:

// functions/package.json
"scripts": {
    "serve": "firebase emulators:start --only functions",

In your terminal you will see the local url for the emulator dashboard, and you can view the functions log. As soon as you update a function, you can test it locally.