how do i exposeexport nested functions to the client?

Return a object from inside the parent function.

const setupProcess = (websocket) => {
    let state = 'idle'
    
    const startProcess = () => {
        state = 'running'

        websocket.onmessage = (event) => {
            const data = JSON.parse(event.data)
            // do something with data
        }
    }

    const stopProcess = () => {
        state = 'idle'
        websocket.onmessage = null
    }
    
    return {
      startProcess,
      stopPorcess
    }
    
}

module.exports = setupProcess

Then you can access it like that:

const setupProcess = require('./setupProcess')
const router = require('./router')

const ws = require('ws')
const app = express()
const port = 3000
connectDatabase(mongoDbUrl)

const ws = new WebSocket('wss//test-websocket.com/ws')
const methods = setupProcess(ws) // initialized

app.use(router(methods))



app.listen(port, () => {
    console.log(`Server is up on port ${port}`)
})

router.js

const express = require('express')
const router = new express.Router()

module.exports = (methods) => {


    router.post('/process/start', async (req, res) => {
        // should be able to startProcess() from here
        //methods.startProcess();
    })

    router.post('/process/start', async (req, res) => {
        // should be able to stopProcess from here
        //methods.stopProcess();
    })

    // return the router
    // so express can use it
    return router;

}