Slack Trigger Node Fails to Process Block Kit Button Payloads Correctly

Based on the error message you’re seeing – “Cannot read properties of undefined (reading ‘type’)” – it appears that the payload coming from Slack when you click a button isn’t in the format the Slack Trigger node expects. In other words, when the node’s code goes to access a field named “type” on the event data, that property is missing (or the entire object is undefined).

What’s Likely Happening

When using interactive components created with Blocks Kit, Slack sends the payload for interactive events in a slightly different way than regular event subscriptions. Often the payload is sent as URL-encoded form data where the actual JSON is contained under a key (usually named “payload”). If that isn’t parsed properly, the Slack Trigger node ends up with an undefined event object, causing the error when it tries to read the “type” property.

How to Investigate and Resolve

1. Verify Payload Format:
   Check how Slack is sending the interactive event. In many cases you may need to inspect the incoming webhook request (for example by logging its full body) to see if the JSON is nested under a “payload” key.

2. Parse the Nested Payload:
   If the interactive event payload is nested as a string inside a “payload” key (a common pattern for Slack interactive messages), you might need to parse that JSON string before the node tries to access its properties.

3. Check Your Slack App Configuration:
   Make sure that your Slack app’s interactive messages configuration is pointing to the correct webhook URL provided by the Slack Trigger node.

   • In your Slack app settings, ensure that the Request URL for interactive components exactly matches the one provided by your node.
   • If Slack is expecting a different format or if the URL isn’t reachable correctly (for example, if you’re toggling between test and production webhook URLs), it might lead to malformed payloads.

4. Compare with a Standard Webhook:
   You mentioned that a standard webhook works the first time. This indicates that when Slack sends the data in a JSON format (as is expected with a standard webhook) the node processes it without error. The discrepancy suggests that interactive events may need additional handling.

Next Steps in n8n

• Review Slack Trigger Node Documentation:
  The Slack Trigger node comes with a list of supported events and parameters. Although interactive components aren’t detailed extensively in the documentation, knowing what structure the node expects can help guide any custom parsing you might need.

• Custom Processing (if necessary):
  If you determine that the payload is indeed coming in nested form (for example, as a URL-encoded “payload” string), you can add a preliminary processing step in your workflow (or in an intermediate function node) to parse that payload before passing it on—for example:

  JAVASCRIPTCopied
  const data = this.getInputData();
  // If the payload is a string under the property 'payload'
  if (data.payload && typeof data.payload === 'string') {
    try {
      data.payload = JSON.parse(data.payload);
    } catch (error) {
      throw new Error('Failed to parse interactive payload');
    }
  }
  return data;

  Adjust this snippet to fit within your workflow as needed.

• Update/Upgrade if Needed:
  Since you’re using n8n version 1.72.1, verify if any updates or bug fixes have been made to the Slack Trigger node that improve support for interactive events in newer releases.

Summary

The error is likely caused by the Slack Trigger node expecting a JSON payload with a top-level “type” property, while the Slack interactive event payload is coming in a different (nested) format. Inspect the payload structure, ensure your Slack app configuration is correct, and add necessary parsing logic if needed. This should resolve the “cannot read properties of undefined” error and allow your interactive button events to be processed correctly by your workflow.

If you continue to have issues, consider reaching out on the n8n community forums or GitHub repository with details of the payload you’re receiving.