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TypeError: err.toJSON is not a function when webhook URL is invalid #511
Description
This took a bit for me to track down (my own mistake), thanks to the real error not being shown in the action. If, for example, I accidentally quote the webhook URL, like:
- name: Notify Slack
uses: slackapi/slack-github-action@v2
with:
errors: true
payload: <redacted>
webhook: ${{ toJSON(secrets.SLACK_WEBHOOK_URL) }}
webhook-type: webhook-triggerI get an exception thrown:
Run slackapi/slack-github-action@v2
Error: TypeError: err.toJSON is not a function
Error: err.toJSON is not a function
TypeError: err.toJSON is not a function
at Webhook.post (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/webhook.js:36:1)
at post (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/send.js:35:1)
at send (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/send.js:15:1)
at file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/index.js:9:1
file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/webhook.js:36
const response = err.toJSON();
^
TypeError: err.toJSON is not a function
at Webhook.post (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/webhook.js:36:1)
at post (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/send.js:35:1)
at send (file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/send.js:15:1)
at file:///home/runner/work/_actions/slackapi/slack-github-action/v2/src/index.js:9:1
It looks like the catch block is assuming the exception will always have a toJSON function, but in my case it obviously doesn't.
The fix of course, was to remove the erroneous toJSON(...) wrapper from the webhook input, but it would have saved me a lot of time if the exception was handled better by thi action.
Note, the err.toJSON in the exception is unrelated to the toJSON in my input - its only coincidental that we both used (different) toJSON functions (JavaScript vs GitHub Expression respectively).
Cheers.
Activity
mwbrooks commentedon Nov 17, 2025
Hi @pcolby 👋🏻
Oof, this must have taken some time to track down. Sorry that we put that on you. 🙇🏻
We've had a few iterations to surface the right level of errors in the GitHub Action and I think you've found an edge-case. Thanks a lot for the details, code example, and resolution. I think this goes a long way in helping us reproduce it and patch it.
@zimeg Since you've made big strides in improving the errors, what are your thoughts on this one?
mwbrooks commentedon Nov 22, 2025
@pcolby I had a chat with @zimeg async and they agree that it's a bug 🐛 So we'll keep this issue open and follow-up with a PR.
Thanks again for taking the time to submit the issue and workaround! 🙇🏻
fix: handle webhook errors without toJSON (fixes slackapi#511)