ack() is not accessible in global middleware in TypeScript

[seratch]

Description

ack() function actually exists in global middleware arguments, but it's not accessible in TypeScript. Note that ack does not exist in middleware args when receiving an Events API payload. It can exist for all the rest.

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• example code related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

---

Bug Report

import { App } from '@slack/bolt';
const app = new App({
  token: process.env.SLACK_BOT_TOKEN,
  signingSecret: process.env.SLACK_SIGNING_SECRET
});

app.use(async ({ body, next, ack }) => {
  if (ack && shouldSkip(body)) {
    await ack();
    return;
  }
  await next();
});

Currently, a workaround is to cast the type to any:

app.use(async (args) => {
  const ack = (args as any).ack;
  if (ack && shouldSkip(args.body)) {
    await ack();
    return;
  }
  await args.next();
});

Reproducible in:

package version: any
node version: any
OS version(s): any

Steps to reproduce:

1. Compile the above code snippet

Expected result:

The ack argument should be available.

Actual result:

Compilation error as below:

[seratch]

Additional note: The ack function can be absent for Events API.

[asontha]

Why is the ack function absent for the Events API?