Slack Events API

The @slack/events-api package helps your app respond to events from Slack's Events API such as new messages, emoji reactions, files, and much more. This package will help you start with convenient and secure defaults.

Deprecation Notice

@slack/events-api officially reached EOL on May 31st, 2021. Development has fully stopped for this package and all remaining open issues and pull requests have been closed.

At this time, we recommend migrating to Bolt for JavaScript, a framework that offers all of the functionality available in those packages (and more). To help with that process, we've provided some migration samples for those looking to convert their existing apps.

Installation

$ npm install @slack/events-api

Usage

These examples show how to get started using the most common features. You'll find even more extensive documentation on the package's website.

Before building an app, you'll need to create a Slack app and install it to your development workspace. You'll also need a public URL where the app can begin receiving events. Finally, you'll need to find the request signing secret given to you by Slack under the "Basic Information" of your app configuration.

It may be helpful to read the tutorials on getting started and getting a public URL that can be used for development. After you have a URL for development, see the section on verifying a request URL for development so you can save it as the Request URL in your app configuration. Now you can begin adding event subscriptions, just be sure to install the app in your development workspace again each time you add new scopes (typically whenever you add new event subscriptions).

Initialize the event adapter

The package exports a createEventAdapter() function, which returns an instance of the SlackEventAdapter class. The function requires one parameter, the request signing secret, which it uses to enforce that all events are coming from Slack to keep your app secure.

const { createEventAdapter } = require('@slack/events-api');

// Read the signing secret from the environment variables
const slackSigningSecret = process.env.SLACK_SIGNING_SECRET;

// Initialize
const slackEvents = createEventAdapter(slackSigningSecret);

Start a server

The event adapter transforms incoming HTTP requests into verified and parsed events. That means, in order for it to emit events for your app, it needs an HTTP server. The adapter can receive requests from an existing server, or as a convenience, it can create and start the server for you.

In the following example, the event adapter starts an HTTP server using the .start() method. Starting the server requires a port for it to listen on. This method returns a Promise which resolves for an instance of an HTTP server once it's ready to emit events. By default, the built-in server will be listening for events on the path /slack/events, so make sure your Request URL ends with this path.

const { createEventAdapter } = require('@slack/events-api');
const slackSigningSecret = process.env.SLACK_SIGNING_SECRET;
const slackEvents = createEventAdapter(slackSigningSecret);

// Read the port from the environment variables, fallback to 3000 default.
const port = process.env.PORT || 3000;

(async () => {
  // Start the built-in server
  const server = await slackEvents.start(port);

  // Log a message when the server is ready
  console.log(`Listening for events on ${server.address().port}`);
})();

Note: To gracefully stop the server, there's also the .stop() method, which returns a Promise that resolves when the server is no longer listening.

const { createServer } = require('http');
const { createEventAdapter } = require('@slack/events-api');
const slackSigningSecret = process.env.SLACK_SIGNING_SECRET;
const slackEvents = createEventAdapter(slackSigningSecret);

// Read the port from the environment variables, fallback to 3000 default.
const port = process.env.PORT || 3000;

// Initialize a server using the event adapter's request listener
const server = createServer(slackEvents.requestListener());

server.listen(port, () => {
  // Log a message when the server is ready
  console.log(`Listening for events on ${server.address().port}`);
});

const { createServer } = require('http');
const express = require('express');
const bodyParser = require('body-parser');
const { createEventAdapter } = require('@slack/events-api');
const slackSigningSecret = process.env.SLACK_SIGNING_SECRET;
const port = process.env.PORT || 3000;
const slackEvents = createEventAdapter(slackSigningSecret);

// Create an express application
const app = express();

// Plug the adapter in as a middleware
app.use('/my/path', slackEvents.requestListener());

// Example: If you're using a body parser, always put it after the event adapter in the middleware stack
app.use(bodyParser());

// Initialize a server for the express app - you can skip this and the rest if you prefer to use app.listen()
const server = createServer(app);
server.listen(port, () => {
  // Log a message when the server is ready
  console.log(`Listening for events on ${server.address().port}`);
});

Listen for an event

Apps register functions, called listeners, to be triggered when an event of a specific type is received by the adapter. If you've used Node's EventEmitter pattern before, then you're already familiar with how this works, since the adapter is an EventEmitter.

The event argument passed to the listener is an object. It's contents corresponds to the type of event its registered for.

const { createEventAdapter } = require('@slack/events-api');
const slackSigningSecret = process.env.SLACK_SIGNING_SECRET;
const slackEvents = createEventAdapter(slackSigningSecret);
const port = process.env.PORT || 3000;

// Attach listeners to events by Slack Event "type". See: https://api.slack.com/events/message.im
slackEvents.on('message', (event) => {
  console.log(`Received a message event: user ${event.user} in channel ${event.channel} says ${event.text}`);
});

(async () => {
  const server = await slackEvents.start(port);
  console.log(`Listening for events on ${server.address().port}`);
})();

Handle errors

If an error is thrown inside a listener, it must be handled, otherwise it will crash your program. The adapter allows you to define an error handler for errors thrown inside any listener, using the .on('error', handlernFn) method. It's a good idea to, at the least, log these errors so you're aware of what happened.

const { createEventAdapter } = require('@slack/events-api');
const slackSigningSecret = process.env.SLACK_SIGNING_SECRET;
const slackEvents = createEventAdapter(slackSigningSecret);
const port = process.env.PORT || 3000;

slackEvents.on('message', (event) => {
  // Oops! This throws a TypeError.
  event.notAMethod();
});

// All errors in listeners are caught here. If this weren't caught, the program would terminate.
slackEvents.on('error', (error) => {
  console.log(error.name); // TypeError
});

(async () => {
  const server = await slackEvents.start(port);
  console.log(`Listening for events on ${server.address().port}`);
})();

Debugging

If you're having difficulty understanding why a certain request received a certain response, you can try debugging your program. A common cause is a request signature verification failing, sometimes because the wrong secret was used. The following example shows how you might figure this out using debugging.

Start your program with the DEBUG environment variable set to @slack/events-api:*. This should only be used for development/debugging purposes, and should not be turned on in production. This tells the adapter to write messages about what its doing to the console. The easiest way to set this environment variable is to prepend it to the node command when you start the program.

$ DEBUG=@slack/events-api:* node app.js

app.js:

const { createEventAdapter } = require('@slack/events-api');
const port = process.env.PORT || 3000;

// Oops! Wrong signing secret
const slackEvents = createEventAdapter('not a real signing secret');

slackEvents.on('message', (event) => {
  console.log(`Received a message event: user ${event.user} in channel ${event.channel} says ${event.text}`);
});

(async () => {
  const server = await slackEvents.start(port);
  console.log(`Listening for events on ${server.address().port}`);
})();

When the adapter receives a request, it will now output something like the following to the console:

@slack/events-api:adapter adapter instantiated - options: { includeBody: false, includeHeaders: false, waitForResponse: false }
@slack/events-api:adapter server created - path: /slack/events
@slack/events-api:adapter server started - port: 3000
@slack/events-api:http-handler request received - method: POST, path: /slack/events
@slack/events-api:http-handler request signature is not valid
@slack/events-api:http-handler handling error - message: Slack request signing verification failed, code: SLACKHTTPHANDLER_REQUEST_SIGNATURE_VERIFICATION_FAILURE
@slack/events-api:http-handler sending response - error: Slack request signing verification failed, responseOptions: {}

This output tells the whole story of why the adapter responded to the request the way it did. Towards the end you can see that the signature verification failed.

If you believe the adapter is behaving incorrectly, before filing a bug please gather the output from debugging and include it in your bug report.

Verify tool

Once you have a URL where you'd like to receive requests from the Events API, you must save it as a Request URL in your Slack app configuration. But in order to save it, your app needs to respond to a challenge request, so that Slack knows its your app that owns this URL. How can you do that if you haven't built the app yet? For development, there is a command line tool built into this package that you can use to respond to the challenge.

Once the package is installed in your app, a command line program will be available in your node_modules directory.

$ ./node_modules/.bin/slack-verify --secret  [--path=/slack/events] [--port=3000]

Run the command with your own signing secret (provided by Slack in the "Basic Information"), and optionally a path and a port. A web server will be listening for requests containing a challenge and respond to them the way Slack expects. Now input input and save the Request URL. Once its saved, you can stop the server with Ctrl-C and start working on your app.

Note: If you're using a tunneling tool like ngrok, the Request URL you save in Slack would be the tunnel URL, such as https://abcdef.ngrok.io, appended with the path. In other words, it should look like https://abcdef.ngrok.io/slack/events. Also make sure that when ngrok was started, it's set to use the port that the tool is listening on. In other words, start ngrok with a command like ngrok http 3000.

More

The documentation website has information about these additional features of the SlackEventAdapter:

• Receiving event envelope and header data
• Custom responses

Examples

• Greet And React - A ready to run sample app that listens for messages and emoji reactions, and responds to them. It is built on top of the Express web framework. It also implements OAuth to demonstrate how an app can handle installation to additional workspaces and be structured to handle events from multiple workspaces.

Requirements

This package supports Node v8 LTS and higher. It's highly recommended to use the latest LTS version of node, and the documentation is written using syntax and features from that version.

Getting Help

If you get stuck, we're here to help. The following are the best ways to get assistance working through your issue:

• Issue Tracker for questions, feature requests, bug reports and general discussion related to these packages. Try searching before you create a new issue.
• Email us in Slack developer support: developers@slack.com
• Bot Developers Hangout: a Slack community for developers building all types of bots. You can find the maintainers and users of these packages in #sdk-node-slack-sdk.