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Omar Houmz
Omar Houmz

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How to create your first Firebase Cloud function

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In this post we will cover one small, but very much hyped, part of serverless; cloud functions. One of the solutions available is firebase's "Firebase Cloud Functions".

We are going to:

  1. Create and set up a Firebase project
  2. Setup a Firebase cloud function
  3. Deploy our first function
  4. Setup a local dev environment


I assume that you have node and npm installed on your machine.

Starter project

Before we start, let us set up a basic project to work with. I created some files and boilerplate code in this repo in the 001-create-a-firebase-function-start folder.

Check the readme file on how to get started with the code.

1. Create and set up a Firebase project

Create a Firebase project

To create a firebase project:

  • Go to the Firebase Console
  • Log in or create your account
  • Once on the home page, click the big "Add project" button
  • Enter a name for your project
  • Click "Continue" and follow the steps

And voila, we have a Firebase project.

Setup our firebase project locally

First, we have to install the Firebase CLI tools:

npm install -g firebase-cli

Then, we login to Firebase from the terminal:

firebase login

This will open a browser tab for us to authenticate to Firebase console. Once authenticated successfully, we should have our terminal prompt back.

Next is to initialize a Firebase project locally:

firebase init functions

The command will guide us through the setup. It will ask about 3 things: the Firebase project (which we already created), Cloud Functions language (I chose JavaScript, you can also choose TypeScript), and whether or not to use ESLint (which I replied with a "yes, please" to).

If all goes well, we should see these files created.

+ .firebaserc
+ .gitignore
+ firebase.json
+ functions/.eslintrc.json
+ functions/.gitignore
+ functions/index.js
+ functions/package-lock.json
+ functions/package.json

In addition to some other files you see, these are the important ones:

  • .firebaserc: stores your project aliases.
  • firebase.json: lists your project configuration.
  • functions/package.json node dependencies for our Firebase cloud function.

The file that we should take a look at now is functions/index.js. This is where we write our code for the cloud function.

Let us use this "helloWorld" example. Notice that I disabled the CORS header since we are in development.

const functions = require('firebase-functions')

exports.helloWorld = functions.https.onRequest((request, response) => {
  // disable CORS header
  response.setHeader('Access-Control-Allow-Origin', '*')

  response.status(200).json({ message: 'And, hello from serverless!' })

3. Deploy our first cloud function

Test the Firebase function locally

Before we deploy, we want to test the function locally. To do so, let us cd into the Functions directory:

cd functions

And start a local emulator (this is provided by the firebase-cli tools):

npm run serve

After this runs, the function will be watched for changes, and the local URL should be logged in the console. Look for something like this:

✔  functions[helloWorld]: http function initialized (http://localhost:5001/YOUR_PROJECT_ID/us-central1/helloWorld).

In the link above, YOUR_PROJECT_ID and us-central1 are the project id and the data center region for your Firebase project respectively. And helloWorld is the function name.

If we visit the URL and all goes well, we see this message:

{ "message": "And, hello from serverless land!" }

Deploy the Firebase cloud function

Now, let us deploy our function. While still being in the functions directory, execute this command:

npm run deploy

The result of the command should have a public URL to trigger our function. It looks something like this:


Trigger the cloud function

Next, let us use the function in our project. To do that, we add a div in the index.html file to show a message:

/* index.html */
  <h1>Hello Firebase Functions</h1>
+ <h2 id="function-response"></h2>

And in the index.js file:

// get the heading element
const messageDiv = document.querySelector('#function-response')

// a function that updates the heading
function updateMessage(message) {
  messageDiv.innerText = message

// make sure to replace these info with yours

// the function trigger endpoint

// trigger the function and get the result back
  .then((result) => result.json())
  // when we have the result, we update the heading with the message
  .then((data) => updateMessage(data.message))

After we made these changes, let us check the result. Run the dev server:

npm run start

If everything went well, we should see in our browser a message like this:

Hello Firebase Functions
And, hello from serverless land!

4. A better local development setup

When we are in development, we don't want to constantly deploy our function to see the result. So, we will leverage the local emulator provided by the Firebase CLI tools to have a better local development setup.

We know how to start the function emulator:

cd functions && npm run serve

We know how to start our dev server:

parcel ./index.html

Now, we need a way to start both of them with one command. Here is the easiest way I could find:

(cd functions && npm run serve) & parcel ./index.html

And to keep this setup, we're gonna edit the scripts in our package.json file:

"scripts": {
-  "start": "parcel ./index.html"
+  "start:frontend": "parcel ./index.html",
+  "start": "(cd functions && npm run serve) & npm run start:frontend"

And now we can use one command to start them both:

npm run start

For the url variable, we should modify it to accomodate for the environment (development vs production):

const url =
  process.env.NODE_ENV === 'production'
    : `http://localhost:5001/${YOUR_PROJECT_ID}/${DATA_CENTER_REGION}/${FUNCTION_NAME}`

And now that we use the local url of the function, we can remove the CORS header in the function:

- // disable CORS header
- response.setHeader('Access-Control-Allow-Origin', '*')
+ // disable CORS header for development
+ if (process.env.NODE_ENV !== 'production') {
+   response.setHeader('Access-Control-Allow-Origin', '*')
+ }

And then deploy the function:

cd functions && npm run deploy


Until this point, we created a Firebase project using the Firebase console. In our local machine, we initialized our project a Firebase function and deployed it. We also configured a better setup for local development.

There is still much we can do here. We will explore other things in the next posts. But, if you are in a hurry, you can check the the Firebase Functions docs.

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