Listening to events

You can listen to any Events API event using the event() method after subscribing to it in your app configuration. This allows your app to take action when something happens in Slack, like a user reacting to a message or joining a channel.

The event() method requires an eventType of type string.

const welcomeChannelId = 'C12345';

// When a user joins the team, send a message in a predefined channel asking them to introduce themselves
app.event('team_join', async ({ event, client, logger }) => {
  try {
    // Call chat.postMessage with the built-in client
    const result = await client.chat.postMessage({
      channel: welcomeChannelId,
      text: `Welcome to the team, <@${event.user.id}>! 🎉 You can introduce yourself in this channel.`
    });
    logger.info(result);
  }
  catch (error) {
    logger.error(error);
  }
});

Filtering on message subtypes

A message() listener is equivalent to event('message')

You can filter on subtypes of events by using the built-in subtype() middleware. Common message subtypes like message_changed and message_replied can be found on the message event page.

// Import subtype from the package
const { App, subtype } = require('@slack/bolt');

// Matches all message changes from users
app.message(subtype('message_changed'), ({ event, logger }) => {
  // This if statement is required in TypeScript code
  if (event.subtype === 'message_changed'
    && !event.message.subtype
    && !event.previous_message.subtype) {
    logger.info(`The user ${event.message.user} changed their message from ${event.previous_message.text} to ${event.message.text}`);
  }
});