Connect your app to the Cloud Functions Emulator

Before connecting your app to the Cloud Functions emulator, make sure that you understand the overall Firebase Local Emulator Suite workflow, and that you install and configure the Local Emulator Suite and review its CLI commands.

Choose a Firebase project

The Firebase Local Emulator Suite emulates products for a single Firebase project.

To select the project to use, before you start the emulators, in the CLI run firebase use in your working directory. Or, you can pass the --project flag to each emulator command.

Local Emulator Suite supports emulation of real Firebase projects and demo projects.

Project type Features Use with emulators

A real Firebase project is one you created and configured (most likely via the Firebase console).

Real projects have live resources, like database instances, storage buckets, functions, or any other resource you set up for that Firebase project.

When working with real Firebase projects, you can run emulators for any or all of the supported products.

For any products you are not emulating, your apps and code will interact with the live resource (database instance, storage bucket, function, etc.).


A demo Firebase project has no real Firebase configuration and no live resources. These projects are usually accessed via codelabs or other tutorials.

Project IDs for demo projects have the demo- prefix.

When working with demo Firebase projects, your apps and code interact with emulators only. If your app attempts to interact with a resource for which an emulator isn't running, that code will fail.

We recommend you use demo projects wherever possible. Benefits include:

  • Easier setup, since you can run the emulators without ever creating a Firebase project
  • Stronger safety, since if your code accidentally invokes non-emulated (production) resources, there is no chance of data change, usage and billing
  • Better offline support, since there is no need to access the internet to download your SDK configuration.

Instrument your app to talk to the emulators

Instrument your app for callable functions

If your prototype and test activities involve callable backend functions, configure interaction with the Cloud Functions for Firebase emulator like this:

        // is the special IP address to connect to the 'localhost' of
        // the host computer from an Android emulator.
        FirebaseFunctions functions = FirebaseFunctions.getInstance();
        functions.useEmulator("", 5001);
iOS - Swift
Functions.functions().useFunctionsEmulator(origin: "http://localhost:5001")

Web version 9

import { getApp } from "firebase/app";
import { getFunctions, connectFunctionsEmulator } from "firebase/functions";

const functions = getFunctions(getApp());
connectFunctionsEmulator(functions, "localhost", 5001);

Web version 8

firebase.functions().useEmulator("localhost", 5001);

Instrument your app for HTTPS functions emulation

Each HTTPS function in your code will be served from the local emulator using the following URL format:


For example a simple helloWorld function with the default host port and region would be served at:


Instrument your app for background-triggered functions emulation

The Cloud Functions emulator supports background-triggered functions from the following sources:

  • Realtime Database emulator
  • Cloud Firestore emulator
  • Authentication emulator
  • Pub/Sub emulator

To trigger background events, connect your app or test code to the emulators using the SDK for your platform.

What other tools for testing Cloud Functions exist?

The Cloud Functions emulator is supplemented by other prototype and test tools:

  • The Cloud Functions shell, which allows for interactive, iterative functions prototyping and development. The shell employs the Cloud Functions emulator with a REPL-style interface for development. No integration with the Cloud Firestore or Realtime Database emulators is provided. Using the shell, you mock data and perform function calls to simulate interaction with products that the Local Emulator Suite does not currently support: Cloud Storage, Pub/Sub, Analytics, Remote Config, Storage, Auth, and Crashlytics.
  • The Firebase Test SDK for Cloud Functions, a Node.js with mocha framework for functions development. In effect, the Cloud Functions Test SDK provides automation atop the Cloud Functions shell.

You can find more about the Cloud Functions shell and Cloud Functions Test SDK at Test functions interactively and Unit testing of Cloud Functions.

What next?