Global middleware

Global middleware is run for all incoming requests before any listener middleware. You can add any number of global middleware to your app by utilizing app.use(fn). The middleware function fn is called with the same arguments as listeners and an additional next function.

Both global and listener middleware must call await next() to pass control of the execution chain to the next middleware, or call throw to pass an error back up the previously-executed middleware chain.

As an example, let's say your app should only respond to users identified with a corresponding internal authentication service (an SSO provider or LDAP, for example). You may define a global middleware that looks up a user record in the authentication service and errors if the user is not found.

// Authentication middleware that associates incoming request with user in Acme identity provider
async function authWithAcme({ payload, client, context, next }) {
  const slackUserId = payload.user;
  const helpChannelId = 'C12345';

  // Assume we have a function that accepts a Slack user ID to find user details from Acme
  try {
    // Assume we have a function that can take a Slack user ID as input to find user details from the provider
    const user = await acme.lookupBySlackId(slackUserId);
      
    // When the user lookup is successful, add the user details to the context
    context.user = user;
  } catch (error) {
    // This user wasn't found in Acme. Send them an error and don't continue processing request
    if (error.message === 'Not Found') {
        await client.chat.postEphemeral({
          channel: payload.channel,
          user: slackUserId,
          text: `Sorry <@${slackUserId}>, you aren't registered in Acme. Please post in <#${helpChannelId}> for assistance.`
        });
        return;
    }
    
    // Pass control to previous middleware (if any) or the global error handler
    throw error;
  }
  
  // Pass control to the next middleware (if there are any) and the listener functions
  // Note: You probably don't want to call this inside a `try` block, or any middleware
  //       after this one that throws will be caught by it. 
  await next();
}