Sending Firebase Cloud Messaging (FCM) Notifications Using Google Apps Script and Web App

Firebase Cloud Messaging (FCM) is a powerful tool that allows you to send notifications to users. In this blog, we will walk through how to create a web app using Google Apps Script that allows you to send FCM notifications by calling a URL with parameters.

Prerequisites 

• A Firebase Project.
• Service Account credentials (client email, private key, and project ID).
• Basic understanding of Google Apps Script.

Steps to Implement the Solution

Here’s how you can set up and deploy a Google Apps Script web app that sends FCM notifications by simply accessing a URL.

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1. Set Up Firebase and Generate Service Account Credentials

Before diving into the code, ensure you have set up Firebase and have your service account credentials ready.

• Go to the Firebase Console.
• Select your project.
• Navigate to Project Settings > Service Accounts.
• Click Generate New Private Key and download the JSON file.
• Note down the client_email, private_key, and project_id.

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2. Update the Script

Below is the JavaScript code that we will use. This script contains two main functions:

• myFunction(): This function generates a JWT access token using your Firebase service account credentials.
• sendMsg(): This function sends an FCM notification using the generated access token.

Script:

var clientemail = "your client email";

var privateKey = "your private key";

var project = "your project id";

function myFunction() {

  var header = `{

    "alg":"RS256",

    "typ":"JWT"

  }`;

  var time = new Date().getTime()/1000;

  var expiration = time + 3600;

var payload = "{'iss':'"+ clientemail+"','scope': 'https://www.googleapis.com/auth/cloud-platform https://www.googleapis.com/auth/firebase.messaging','aud' : 'https://oauth2.googleapis.com/token','exp':"+expiration+",'iat' :"+ time+"}";

var base64Header = Utilities.base64Encode(header).replace(['+','/','='],['-','_','']);

// console.log(payload);

var base64Payload = Utilities.base64Encode(payload).replace(['+','/','='],['-','_','']);

var signatureInput = base64Header+"."+base64Payload;

var signature = Utilities.computeRsaSha256Signature(signatureInput,privateKey);

var base64UrlSignature = Utilities.base64Encode(signature).replace(['+','/','='],['-','_','']);

var jwt = base64Header+'.'+base64Payload+'.'+base64UrlSignature;

var payload2 = {

       'grant_type' : 'urn:ietf:params:oauth:grant-type:jwt-bearer',

    'assertion' : jwt

  };

 

  var url ='https://oauth2.googleapis.com/token';

  // Options for the POST request

  var options = {

    'method' : 'post',

    'contentType': 'application/json',

//    'headers': headers,

    'muteHttpExceptions':true,

    'payload' : JSON.stringify(payload2)

  };

 

  // Make the POST request

  var response = UrlFetchApp.fetch(url, options);

// console.log(response.getContentText());

var json = JSON.parse(response.getContentText());

console.log(json["access_token"]);

return json["access_token"];

 

}

function sendMsg(title,msg) {

 

  var message = {

    "message": {

      "topic": "all",

      "notification": {

        "title": title,

        "body": msg

      },

      "data":{

      }

    }

  };

  var options = {

    headers: {

     

      "Content-Type": "application/json; charset=UTF-8",

      "Authorization": "Bearer " + myFunction()

    },

     method:"post",

    payload: JSON.stringify(message),

   muteHttpExceptions :true

  }

console.log(options)

  var resp = UrlFetchApp.fetch('https://fcm.googleapis.com/v1/projects/' + project + '/messages:send', options);

console.info(resp.getContentText())

if(resp.getContentText().includes("error"))

  return JSON.parse(resp.getContentText())["error"]["message"];

return "Message sent successfully";

}

function doGet(e){

    try{

      var title = e.parameter.title;

      var msg = e.parameter.msg;

    }catch{

      var title = "test";

      var msg = "test msg";

    }

   return ContentService.createTextOutput().append(sendMsg(title,msg)).setMimeType(ContentService.MimeType.TEXT);

}

Explanation of the Code:

• myFunction(): This function creates a JWT token using Firebase credentials (clientemail and privateKey), which is then used to generate an access token for authentication.
• sendMsg(): This function sends the FCM message to all devices subscribed to the "all" topic. The message payload includes the title and body of the notification.
• doGet(): This function allows you to pass title and msg as URL parameters, so you can easily trigger a notification by visiting a URL.

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3. Deploy the Script as a Web App

To deploy the script as a web app, follow these steps:

Step 1: Save the Script

• Once the script is written, go to File > Save to save your project.

Step 2: Deploy the Web App

• Click on Deploy > New deployments > Select Type: Web App.
• Under Execute as, choose Me.
• Under Who has access, select Anyone.
• Click Deploy and note down the generated web app URL.

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4. Send Messages via URL

You can now send FCM messages by accessing the deployed web app URL with query parameters for the title and message. For example:

perl
https://script.google.com/macros/s/your-web-app-id/exec?title=Hello&msg=Welcome to FCM Notifications

This URL will trigger the doGet() function, which sends a notification with the title Hello and the message Welcome to FCM Notifications to all devices subscribed to the all topic.

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5. Testing and Debugging

• Open the URL in a browser with different titles and messages to test if the FCM messages are sent successfully.
• Use the Firebase Console to verify whether notifications are received by the devices.

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Conclusion

Using Google Apps Script as a web app to send Firebase Cloud Messaging notifications is an efficient way to handle notifications without the need for complex server-side solutions. By simply accessing a URL with query parameters, you can send notifications to all devices subscribed to a specific FCM topic.

Now you have an easy-to-deploy, lightweight system for sending notifications via FCM. This method is ideal for lightweight applications or automation needs.