JavaScript function call context has valid parameters, but function doesn't get them

In a custom event-handling setup using callbacks, the calling context of the callback shows valid values for the callback parameters, but the argument values are undefined after stepping into the function.

In case it matters, this is inside of an Electron renderer process which uses React.

In a vanilla JavaScript object, the debugger shows expected contextual values for everything on the line of the callback reporting SerialPortEvent.Opened.

// file: serialPort.js (no imports)
class SerialPortEvent {
  Discovered = 0
  Opened = 1
  Closed = 2
  DataReceived = 3
  Error = 4
}

class SerialPortSpec {
...
  addEventListener(listener) {
    this.listeners.push(listener)
  }

  open(baudRate) {
    const baudRateInt = parseInt(baudRate, 10)

    window.serialport.open(this.portInfo.path, baudRateInt)
      .then(binding => {
        this.baudRate = baudRateInt
        this.binding = binding

        // Here, the debugger shows all locals are as expected

        this.listeners.forEach(listener => listener(SerialPortEvent.Opened))
      }).catch(reason => {
        this.listeners.forEach(listener => listener(SerialPortEvent.Error, reason))
      })
  }
...
}

...

module.exports = { SerialPortEventHub, SerialPortSpec, SerialPortEvent }

However, one "step into" later, at the beginning of the callback function "portEventHandler", the argument values are undefined. I would love to know why! Thanks.

// file SerialPortUI.jsx

import React from 'react'
import { SerialPortEventHub, SerialPortEvent, SerialPortSpec } from './serialPort'
import './serialportui.css'

class SerialPortPanel extends React.Component {
...
  portEventHandler(eventType, ...data) {
    // Here, eventType is undefined. This file does import SerialPortEvent.
    switch (eventType) {
      ...
    }
  }
...
}

> class SerialPortEvent {
> Discovered = 0
> Opened = 1
> Closed = 2
> DataReceived = 3
> Error = 4
> }

That's not a proper enum declaration. Class fields will be created on an instance, the static class property SerialPortEvent.Opened you are accessing in your code is indeed undefined. And that's the value the called function receives.

Instead, you'll want to use an object with properties:

const SerialPortEvent = {
  Discovered: 0,
  Opened: 1,
  Closed: 2,
  DataReceived: 3,
  Error: 4,
};

Never use class syntax if you don't ever instantiate it (use new)!