"Path must be a string" when combining apps

[markjm]

Bug Report

To reproduce:

1. set up an empty/new probot app with the package.json set up as described here
   https://probot.github.io/docs/deployment/#combining-apps
2. Run yarn start
   Current Behavior
   Error:

yarn start           
yarn run v1.22.18
$ probot run
/Users/../code/../node_modules/resolve/lib/sync.js:75
        throw new TypeError('Path must be a string.');
              ^

TypeError: Path must be a string.
    at resolveSync (/Users/../code/../node_modules/resolve/lib/sync.js:75:15)
    at Object.resolveAppFunction (/Users/../code/../node_modules/probot/lib/helpers/resolve-app-function.js:30:23)
    at combinedApps (/Users/../code/../node_modules/probot/lib/run.js:101:56)
    at async Server.load (/Users/../code/../node_modules/probot/lib/server/server.js:52:9)

Expected behavior/code
Expect the app to start and work as expected.

Creating an empty index.js and running yarn start ./index.js works fine.

Environment

• Probot version(s): 12.2.0
• Node/npm version: v16/yarn v1

Possible Solution
Guard the below line with existance check of appPath

probot/src/run.ts

Line 136 in 420f886

const appFn = await resolveAppFunction(appPath);

or update docs to require inclusion of index.js

Happy to send PR if consensus on problem & my proposed solution. Thanks!

[welcome]

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[joshbranham]

I ran into this issue as well, in my case I am using the multi-app method to just have a shell around deploying the settings-app Probot.

I have an app.js like:

const settings = require("probot-settings");

module.exports = async (app, options) => {
  await settings(app, options);
};

And then to get things to work in NODE_ENV=production I had to use probot run ./app.js but in local environments probot run works.