TypeError: Cannot read properties of undefined (reading 'type') at SocketModeClient.onWebSocketMessage

Sometimes we get an error due to event.payload.event being undefined

https://github.com/slackapi/node-slack-sdk/blob/039bd050cd736d666d063397067cb014adf23711/packages/socket-mode/src/SocketModeClient.ts#L344

TypeError: Cannot read properties of undefined (reading 'type')
    at SocketModeClient.onWebSocketMessage (/app/node_modules/@slack/socket-mode/dist/SocketModeClient.js:620:43)
    at WebSocket.onMessage (/app/node_modules/ws/lib/event-target.js:132:16)
    at WebSocket.emit (node:events:514:28)
    at Receiver.receiverOnMessage (/app/node_modules/ws/lib/websocket.js:1070:20)
    at Receiver.emit (node:events:514:28)
    at Receiver.dataMessage (/app/node_modules/ws/lib/receiver.js:517:14)
    at Receiver.getData (/app/node_modules/ws/lib/receiver.js:435:17)
    at Receiver.startLoop (/app/node_modules/ws/lib/receiver.js:143:22)
    at Receiver._write (/app/node_modules/ws/lib/receiver.js:78:10)
    at writeOrBuffer (node:internal/streams/writable:447:12)
    at _write (node:internal/streams/writable:389:10)
    at Receiver.Writable.write (node:internal/streams/writable:393:10)
    at TLSSocket.socketOnData (/app/node_modules/ws/lib/websocket.js:1164:35)
    at TLSSocket.emit (node:events:514:28)

Packages:

Select all that apply:

• @slack/web-api
• @slack/rtm-api
• @slack/webhooks
• @slack/oauth
• @slack/socket-mode
• @slack/types
• I don't know

Reproducible in:

The Slack SDK version

"@slack/socket-mode": "1.3.6"

Node.js runtime version

v20.9.0

OS info

#141~20.04.1-Ubuntu SMP Thu Jan 16 18:38:51 UTC 2025

Steps to reproduce:

N/A

Expected result:

Missing payload handled gracefully

Actual result:

Unhandled error

Requirements

For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. 🙇

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

Start in packages/socket-mode/src/SocketModeClient.ts at the referenced line and trace how messages with an undefined event.payload.event are handled. Check the socket-mode package's existing test coverage for WebSocket messages, then verify that a missing payload no longer causes an unhandled TypeError and is handled gracefully.