Overriding callbackOptions while keeping default behavior

You can override the callbackOptions and provide additional logic to run before/after installing the Slack app.

We use it to send a welcome message to the installing user after a succesfull installation. However, you need to manually end the response using res.end:

callbackOptions: {
        success: (installation, _options, _req, res) => {
          app.client.chat.postMessage({
            token: installation.bot.token,
            channel: installation.user.id,
            text: 'Welcome to my app!'
          });

          res.end('Success!');
        }
      }

This example just displays a basic message, rather than the default Slack page that displays when no logic is provided, that also automatically redirects to the Slack app, along with a magic link:

Requirements

I want the callback to provide a default, that you can call after executing your custom logic, which will show the same page but will still allow me to run my logic. The Python SDK already has this; see this PR.

Start by tracing how callbackOptions and the success callback are handled during Slack app installation in bolt-js. Compare the desired behavior with the referenced bolt-python PR. Done means custom success logic can run while preserving the default success page, redirect, and magic link without requiring callers to end the response manually.