Firebase functions not recognizing express app routes outside of index.ts

I recently deployed a TypeScript project to firebase-functions with everything in index.ts. Everything worked but when I started to refactor my code I soon realized firebase does not recognize routes specified in files outside of index.ts (or the file where you wrap your express app in functions.https.onRequest()).

I have replicated the issue below:

//index.ts    
import * as functions from 'firebase-functions'; 
import * as express from "express";

export const expressApp = express();

expressApp.get("/hi", (req, res)=> {
   res.send("hi");
})

export const TestApp = functions.https.onRequest(expressApp);

External file in same directory that adds the "/hello" route.

//external.ts
import { expressApp } from "./index";

expressApp.get("/hello", (req, res)=> {
    res.send("hello")
})

After I use firebase-deploy, the "/hi" route in index.ts works but the "/hello" route in external.ts does not (Cannot GET /hello).

My guess is that the TestApp is being exported to firebase-functions before it's called by any external files.

Is there any way to get around this in a TypeScript project?

When you start your project, the code inside index.ts is executed, but as it doesn't reference external.ts at all, that code is never executed.

What you can do instead is call external.ts from inside index.ts and then invoke it by passing in the express app you would like to modify.

//index.ts    
import * as functions from 'firebase-functions'; 
import * as express from "express";
import {attachRoutes: attachExternalRoutes} from "./external";

export const expressApp = express();

expressApp.get("/hi", (req, res)=> {
   res.send("hi");
})

attachExternalRoutes(expressApp); // attaches routes in external

export const TestApp = functions.https.onRequest(expressApp);

//external.ts
export function attachRoutes(expressApp) {
  expressApp.get("/hello", (req, res)=> {
    res.send("hello")
  })
}