Listener middleware

Listener middleware is used for logic across many listener functions (but usually not all of them). They are added as arguments before the listener function in one of the built-in methods. You can add any number of listener middleware before the listener function.

There’s a collection of built-in listener middleware that you can use like directMention which filters out any message that doesn’t directly @-mention your bot at the start of a message.

But of course, you can write your own middleware for more custom functionality. While writing your own middleware, your function must call await next() to pass control to the next middleware, or throw to pass an error back up the previously-executed middleware chain.

As an example, let’s say your listener should only deal with messages from humans. You can write a listener middleware that excludes any bot messages.

// Listener middleware that filters out messages with 'bot_message' subtype
async function noBotMessages({ message, next }) {
  if (!message.subtype || message.subtype !== 'bot_message') {
    await next();
  }
}

// The listener only receives messages from humans
app.message(noBotMessages, async ({ message, logger }) => logger.info(
  // Handle only newly posted messages
  if (message.subtype === undefined
    // || message.subtype === 'bot_message'
    || message.subtype === 'file_share'
    || message.subtype === 'thread_broadcast') {
    logger.info(`(MSG) User: ${message.user} Message: ${message.text}`)
  }
));