How to Build a Slack App

Slack Apps, or Bots, allow you to extend slack with interactive features that can improve your teams productivity. The following lesson is a step-by-step guide to building a Slack App using Firebase Cloud Functions as the backend server.

Our Slack App will perform the following tasks.

• Listen to events, such as a new user joining the #general channel.
• Retrieve the user’s slack profile.
• Send a private personalized message.
• Add a slash command for user-directed actions.

Create a Slack App

At this point, it is assumed you have admin access to a Slack workspace. If not, feel free to create one to follow this tutorial.

Once you have a workspace, create a new Slack App.

Verify Cloud Function Ownership

Before Slack can send events to our Cloud Function, it needs to verify that we own the server. Slack performs a handshake by sending an HTTP request with a challenge parameter to the Cloud Function, then the function must respond back with the same value.

Initialize Cloud Functions

Initialize Cloud Functions. This demo uses the TypeScript flavor.

firebase init functions

Build The Challenge Function

The function only needs to respond with the challenge one time.

export const myBot = functions.https.onRequest( (req, res) => {
  
  // Request from Slack
  const { challenge }  = req.body;

  // Response from You
  res.send({ challenge })

});

Deploy it

firebase deploy --only functions:myBot

Enter The Deployed URL

Subscribe to an event that you want to listen to, then paste in the Function URL from the last step. Slack should automatically verify the URL and give it a green checkmark ✅.

Build the Bot

Now it’s time to do some real work.

Install Dependencies

The Slack Node SDK is a monorepo that contains several packages. The Web API can read and modify data in the workspace. Google PubSub will be used to handle long running background tasks in te following steps.

npm install @slack/web-api
npm install @google-cloud/pubsub

OAuth Token

The OAuth token is used to authenticate your bot/server into a workspace so it can interact with your channel (like post messages).

Once installed, it will take you directly to the OAuth token. It usually starts with xoxb or xoxp.

Copy the OAuth Token and save it as a Firebase Functions environment variable.

firebase functions:config:set slack.token=YOUR-TOKEN

Signing Secrets

When receiving events from Slack, you should validate the signing secret, which can be found in Basic Info panel. This ensures that only requests from Slack can interact with your function by decoding the digital signature of the request.

firebase functions:config:set slack.signing_secret=YOUR-TOKEN

Add Scopes (Permissions)

OAuth scopes define what your app is allowed to do. You can fine tune permissions under the OAuth & Permissions page. Follow the principle of least privilege and only allow your bot access to resources that it actually needs to do its job. In our case, we need to read a user profile and add them to a specific slack channel.

Listen to Events

Our first goal is to listen to events that happen in the Slack workspace. Slack should notify our server anytime a user joins a channel with the member_joined_channel event.

⚡ Your server must respond be quickly, within 3000ms or less, otherwise Slack will timeout and attempt to retry.

So then, how do we build an app that performs a long-running backend process? There are many right answers, but in Firebase, the best option is to enqueue a PubSub Cloud Function. It allows the initial HTTP endpoint to simply hand off the message and respond quickly to Slack.

Import the dependencies and initlizize them with the environment credentials.

import * as functions from 'firebase-functions';

import { WebClient } from '@slack/web-api';
const bot = new WebClient(functions.config().slack.token);

const { PubSub } = require('@google-cloud/pubsub');
const pubsubClient = new PubSub();

HTTP Gateway

The HTTP gateway validates the request, enqueues a PubSub message with the request, then responds with a 200 code to keep Slack happy.

Extra Snippet: verifySlackSignature

export const myBot = functions.https.onRequest( async (req, res) => {
    
    // Validate Signature
    verifySlackSignature(req); // See snippet above for implementation

    const data = JSON.stringify(req.body);
    const dataBuffer = Buffer.from(data);

    await pubsubClient
            .topic('slack-channel-join')
            .publisher()
            .publish(dataBuffer);


    res.sendStatus(200);

});

PubSub Function

All the heavy-lifting happens in the PubSub function because we have no time-constraints here. The message.json contains the same data you would have handled in req.body in the HTTP function.

  export const slackChannelJoin = functions.pubsub.topic('slack-channel-join')
    .onPublish(async (message, context) => {


    const { event } = message.json; 

    const { user, channel } = event;

    // TODO something cool...


});

Respond To the User as the Bot

In this section, our app makes requests to the Slack API that (1) fetch the user’s Slack profile (2) invite them to a channel, and (3) greet them with a direct message.

Note: The Slack API does not provide typings for the response object, so you’ll have to treat it as any in typescript.

export const slackChannelJoin = functions.pubsub
  .topic('slack-channel-join')
  .onPublish(async (message, context) => {

    const { event } = message.json; 

    const { user, channel } = event;

    // IDs for the channels you plan on working with
    const generalChannel = 'C12345';
    const newChannel = '#froopy-land';

    // Throw error if not on the general channel
    if (channel !== generalChannel) {
        throw Error()
    }


  // Get the full Slack profile

    const userResult = await bot.users.profile.get({ user });
    const { email, display_name } = userResult.profile as any;


    // Invite the slack user to a new channel
    const invite = await bot.channels.invite({
        channel: newChannel,
        user
    });


    // Send a Message
    const chatMessage = await bot.chat.postMessage({
        channel: newChannel,
        text: `Hey ${display_name}! So glad to have you on my Slack!`
    });


});

Additional Ideas to Try

Listen to Slash Commands

Events are great, but somethings you want to give users tools to manually kick off interactivity - that’s where slash commands come in. They work very similar to events, but are are triggered by the user entering /some-command into the workspace.

/slap someone

Learn more about slack commands in the full lesson by AJonP.

Add a Conversational Bot

DialogFlow can integrate into a Slack App with just a few button clicks. Turning your app into a full-blown chatbot allows users to have human-like conversations with the app, which is often more natural than slash commands.