What causes "Tried add listener but no valid callback function specified." in quasarframework/quasar?

Pass a function as the second argument to on().

Tried add listener but no valid callback function specified.

Error message

Tried add listener but no valid callback function specified.

What it means

BexBridge.on(event, callback) received a callback that is not a function. The bridge warns and does not register the listener, so messages for that event will silently never be handled.

Source

Thrown at app-vite/exports/bex/private/bex-bridge.js:268

    delete this.portMap.background
    this.isConnected = false
    // an explicit disconnect also opts out of the auto-reconnect behavior
    this.#wasConnected = false
    return Promise.resolve()
  }

  /**
   * @param {string} event
   * @param {(message: Message) => void} callback
   */
  on(event, callback) {
    if (!event) {
      this.warn('Tried add listener but no event specified.')
      return
    }

    if (typeof callback !== 'function') {
      this.warn('Tried add listener but no valid callback function specified.')
      return
    }

    const target = (this.listeners[event] ||= [])
    target.push({ type: 'on', callback })
    this.log(`Added a listener for event: "${event}".`)
  }

  /**
   * @param {string} event
   * @param {(message: Message) => void} callback
   */
  once(event, callback) {
    if (!event) {
      this.warn('Tried add listener but no event specified.')
      return
    }

View on GitHub (pinned to 4841521b5f)

Solutions

Pass a function as the second argument to on().
Verify the handler import is correct and not undefined.
If subscribing conditionally, still supply a no-op function rather than omitting the argument.

Example fix

// before
bridge.on('message', this.handler) // this.handler is undefined
// after
const handler = this.handler ?? (() => {})
bridge.on('message', handler)

Defensive patterns

Strategy: type-guard

Type guard

const isFn = (f) => typeof f === 'function'
if (isFn(callback)) bridge.on(event, callback)

Prevention

Always pass a function reference as the 2nd argument.
Verify handler imports resolve to defined functions.
Use no-op fallbacks for optional handlers: (handler ?? (() => {})).

When it happens

Trigger: bridge.on('my-event'), bridge.on('my-event', undefined), or passing something like an object/array/string as the second argument.

Common situations: Misremembering the signature (passing options as 2nd arg); a handler imported as undefined due to wrong named import; refactoring that removed the handler but left the registration.

Related errors

AI-assisted analysis of quasarframework/quasar@4841521b5f (2026-08-30). Data as JSON: /api/errors/664f172af154e1d7. Report an issue: GitHub.