Add Firebase Functions integration guide

_includes/icons/firebase.html

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+<svg fill="{{include.color}}" width="{{include.width}}" height="{{include.width}}" role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>Firebase</title><path d="M3.89 15.672L6.255.461A.542.542 0 017.27.288l2.543 4.771zm16.794 3.692l-2.25-14a.54.54 0 00-.919-.295L3.316 19.365l7.856 4.427a1.621 1.621 0 001.588 0zM14.3 7.147l-1.82-3.482a.542.542 0 00-.96 0L3.53 17.984z"/></svg>

_includes/layouts/docs_nav.html

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     </li>
     <li><a href="/docs/integrations/docker-compose/express">Express</a></li>
     <li><a href="/docs/integrations/docker-compose/rails">Rails</a></li>
+    <li>
+      {% include layouts/docs_nav_title_grid.html name="Firebase Functions" icon="firebase" color="#FFCA28" %}
+    </li>
+    <li><a href="/docs/integrations/firebase-functions/nodejs">Node.js</a></li>
     <li>
       {% include layouts/docs_nav_title_grid.html name="Fly.io" icon="fly" color="#7b3BE2" %}
     </li>

docs/integrations/firebase-functions/nodejs.md

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+---
+layout: docs
+title: "Firebase Functions with Node.js - Integrations"
+---
+
+{% include helpers/reading_time.html %}
+
+{% include icons/firebase.html width="50" color="#FFCA28" %}
+{% include icons/nodejs.html width="50" color="#339933" %}
+
+##### Integrations
+
+# __Firebase Functions with Node.js__
+
+Learn how to configure Firebase Functions with Dotenv Vault in a simple Node.js web app. This tutorial assumes you are already familiar with `.env` files and know [how to sync them](/docs/tutorials/sync).
+
+You can find a complete [example repo here](https://github.com/dotenv-org/integration-example-firebase-functions-nodejs).
+
+## Initial setup
+Create a new Firebase Functions setup, if you don't have one already, or use an existing one to get started.
+
+You can initialize a basic setup by calling `firebase` in your project's `root` folder.
+
+##### CLI
+
+```shell
+firebase init
+```
+
+## Package installation
+Hop into your newly created `functions` folder and install the [`dotenv-vault-core`](https://github.com/dotenv-org/dotenv-vault-core) package with `npm`.
+
+##### CLI
+```shell
+npm install dotenv-vault-core --save
+```
+
+Open the `index.js` file that Firebase created for you and add a reference for the `dotenv-vault-core` package just after the `firebase-functions` require declaration.
+
+Reference the `defineString` method of the `firebase-functions/params` package and declare a `const` to store `DOTENV_KEY` as the name of the key you will be using to connect to `Dotenv Vault` later on.
+
+Lastly, add a `runWith` method to the Firebase Function you will be exporting and add the `const` you created earlier as a parameter. Here's how the `index.js` file in your `functions` folder could look like:
+
+##### Node.js
+
+```js
+// ./functions/index.js
+const functions = require("firebase-functions");
+require('dotenv-vault-core').config()
+console.log(process.env) // remove this after you've confirmed it is working
+
+const { defineString } = require('firebase-functions/params');
+const dotenvKey = defineString('DOTENV_KEY');
+
+exports.helloWorld = functions.runWith({vaultKey: dotenvKey}).https.onRequest((request, response) => {
+  functions.logger.info("Hello logs!", {structuredData: true});
+  response.send("Hello from Firebase!");
+});
+
+```
+[Example](https://github.com/dotenv-org/integration-example-firebase-functions-nodejs/blob/main/functions/index.js).
+
+## Build the Vault
+Now that you have Firebase Functions set up, confirm you are logged in and your Vault is synced locally by running `npx dotenv-vault pull development`. Once ready, proceed by building your Vault with `npx dotenv-vault build`.
+
+##### CLI
+
+```shell
+npx dotenv-vault build
+```
+
+Once Vault has finished building, it will provide you with access to its decryption keys, which you can use to interact with protected environment variables with ease.
+
+To retrieve a key, just input `npx dotenv-vault keys`, followed by your preferred environment, like `development`, for example. You can do the same with other environments such as `ci` and `production.`
+
+The outcome of this will be a long URI being returned. You will immediately recognize it as it always starts with `dotenv://:key` and ends in `?environment=` with the environment you have chosen.
+
+##### CLI
+
+```shell
+npx dotenv-vault keys development
+remote:   Listing .env.vault decryption keys... done
+
+dotenv://:[email protected]/vault/.env.vault?environment=development
+```
+
+## Set deployment
+With the decryption key safely in your possession, it is time to navigate to deploy your function via the CLI. Make sure you first run it with `--only functions` as parameters, so Firebase Functions can update its environment variable settings.
+
+##### CLI
+
+```shell
+firebase deploy --only functions
+```
+
+Once Firebase Functions finishes preparing your `codebase`, it will pause the `build` process to ask you to enter the value of the `DOTENV_KEY` you declared earlier. Enter the `Vault` decryption key as its value and confirm to continue building.
+
+{% include helpers/screenshot.html url="https://res.cloudinary.com/dotenv-org/image/upload/v1671086649/integrations/dotenv_vault_firebase_functions_environment_variable_settings_yfyxrf.png" %}
+
+## Commit and push
+
+That's it!
+
+Commit those changes safely to code and deploy your Firebase Functions without the need for the `--only functions` parameter.
+
+When the build runs, it will recognize the `DOTENV_KEY`, decrypt the `.env.vault` file, and load the `development` environment variables to `ENV`.
+
+If a `DOTENV_KEY` is not set when developing on local machine, for example, it will fall back to standard `Dotenv` functionality.
+
+You'll know things worked correctly when you see `'Loading .env from encrypted .env.vault'` in your Firebase Functions logs.
+
+{% include helpers/screenshot.html url="https://res.cloudinary.com/dotenv-org/image/upload/v1671086649/integrations/dotenv_vault_firebase_functions_logs_encrypted_loading_env_vault_l3ewwx.png" %}