How can I access the Express app instance to set CORS origin in a Probot app?

Question

The probot documentation mentions that I can use routes just like I would in a vanilla Express server.

I wantr to set CORS origin headers for these routes. In a vanilla Express server I would use the cors package:

const cors = require('cors')

...

app.use(cors())

But the probot app does not have the function use.

module.exports = app => {
  app.use(cors(corsOptions));
// ...

causes the error:

ERROR (event): app.use is not a function
    TypeError: app.use is not a function

How do I set CORS?

Answer 1

You must start the app programmatically. This way you can access the Express app AFTER probot loads but BEFORE Probot starts running:

const cors = require("cors");
const bodyParser = require("body-parser");
const { Probot } = require("probot");
const { corsOptions } = require("./src/util/init-server.js");
const endpoint = require("./src/controller/endpoint");
const { handleWhatever } = require("./src/controller/controller");


// https://github.com/probot/probot/blob/master/src/index.ts#L33
const probot = new Probot({
  id: process.env.APP_ID,
  port: process.env.PORT,
  secret: process.env.WEBHOOK_SECRET,
  privateKey: process.env.PRIVATE_KEY,
  webhookProxy: process.env.WEBHOOK_PROXY_URL,
});


const probotApp = app => {
  /** Post a comment on new issue */
  app.on("issues.opened", async context => {
    const params = context.issue({ body: "Hello World!" });
    return context.github.issues.createComment(params);
  });

  /** --- Express HTTP endpoints --- */
  const router = app.route("/api");
  router.use(cors(corsOptions)); // set CORS here
  router.use(bodyParser.json());
  router.use(bodyParser.urlencoded({ extended: true }));
  // router.set("trust proxy", true);
  // router.use(require('express').static('public')); // Use any middleware
  router.get("/ping", (req, res) => res.send("Guten Tag! " + new Date()));
  router.post(endpoint.handleWhatever , handleWhatever );
};


/** --- Initialize Express app by loading Probot --- */
probot.load(probotApp);

/* ############## Express instance ################ */
const app = probot.server;
const log = probot.log;
app.set("trust proxy", true);

/** --- Run Probot after setting everything up --- */
Probot.run(probotApp);

Here some GitHub issues and docs that helped me answer my question:

• probot index.js https://github.com/probot/probot/blob/master/src/index.ts#L33
• Alternate way of running a probot app @see https://probot.github.io/docs/development/#alternate-way-of-running-a-probot-app
• Programmatic access for starting probot @see https://github.com/probot/friction/issues/8
• Programmatic start @see https://github.com/probot/probot/issues/794
• Access to Express app object @see https://github.com/probot/probot/issues/878
• HTTP routes @see https://probot.github.io/docs/http/