Code on GitHub Slack Platform Home

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.

Installation

$ npm install @slack/events-api

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 tutorial on developing Slack apps locally. 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.

Using an existing HTTP server

The event adapter can receive requests from an existing Node HTTP server. You still need to specify a port, but this time its only given to the server. Starting a server in this manner means it is listening to requests on all paths; as long as the Request URL is routed to this port, the adapter will receive the requests.

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}`);
});
Using an Express app

The event adapter can receive requests from an Express application. Instead of plugging the adapter’s request listener into a server, it’s plugged into the Express app. With Express, app.use() can be used to set which path you’d like the adapter to receive requests from. You should be careful about one detail: if your Express app is using the body-parser middleware, then the adapter can only work if it comes before the body parser in the middleware stack. If you accidentally allow the body to be parsed before the adapter receives it, the adapter will emit an error, and respond to requests with a status code of 500.

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 <signing_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.


Receive additional event data

The adapter can trigger listeners with more data than only the event body. The listeners can receive additional arguments: the event envelope, and the request headers.

The envelope contains data regarding how the event was triggered, in addition to the event itself. In order to receive this data in listeners, the adapter must be initialized with the includeBody option set to true. All listeners will now be triggered with an additional argument which contains the envelope.

The headers contain data regarding whether the event is a retry of a previously failed delivery. In order to receive this data in listeners, the adapter must be initialized with the includeHeaders option set to true. All listeners will now be triggered with an additional argument which contains an key-value object describing the HTTP request headers.

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

// Initialize the adapter to trigger listeners with envelope data and headers
const slackEvents = createEventAdapter(slackSigningSecret, {
  includeBody: true,
  includeHeaders: true,
});

// Listeners now receive 3 arguments
slackEvents.on('message', (event, body, headers) => {
  console.log(`Received a message event: user ${event.user} in channel ${event.channel} says ${event.text}`);
  console.log(`The event ID is ${body.event_id} and time is ${body.event_time}`);
  if (headers['X-Slack-Retry-Num'] !== undefined) {
    console.log(`The delivery of this event was retried ${headers['X-Slack-Retry-Num']} times because ${headers['X-Slack-Retry-Reason']}`);
  }
});

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

Note: If the includeHeaders option is set to true, but the includeBody argument is not, then listeners will receive only 2 arguments: event and headers.


Custom responses

The adapter allows your listener to determine whether exactly how it should respond to the incoming HTTP request. This is an advanced feature, and should not be used unless you have a specific need such as: turning event delivery retries off, redirecting Slack to deliver the event to another URL, or changing the HTTP response body.

In order to customize responses, the adapter must be initialized with the waitForResponse option set to true. Once this option is set, listeners will be triggered with an additional respond() argument that every listener must call in under 3 seconds. When the event was handled normally, call respond() with no arguments. If there was an error, call respond(error) with an object that has a status property set to a valid HTTP status code. If you’d like to turn event deliveries off, call respond(null, options) with an object that has the failWithNoRetry property set to true. If you’d like to redirect, call respond(null, options) with an object that has the redirectLocation property set to the URL. Lastly, if you’d like to customize the respond body, call respond(null, options) with an object that has the content property set to a string you’d like to use as the body.

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

// Initialize the adapter to trigger listeners with the respond function
const slackEvents = createEventAdapter(slackSigningSecret, {
  waitForResponse: true,
});

// Redirect events of 'message' type to another URL
slackEvents.on('message', (_event, respond) => {
  respond(null, {
    redirectLocation: 'https://example.com/slack/events/message',
  });
});

// Its now required to call the respond function in every listener
slackEvents.on('reaction_added', (event, respond) => {
  console.log('Reaction event received');
  // Normal success
  respond();
})

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