Why it happens
The release changed the shape of some data or a component's props, and the code reads a field that is no longer there — “Cannot read properties of undefined” is the usual wording.
How to tell
- One fingerprint, many occurrences, first seen at the deploy time.
- The first occurrence just after the publish time.
- The files changed in the publish touch that page.
The fix
- If the page matters now, roll back to the last verified publish; fix forward otherwise.
- Guard the read: handle the missing field, and make the loader return the shape the page expects in every branch.
- Add the case to the journey that covers that page, so the next publish is checked for it.
The prompt for your builder
Paste this into Lovable, Bolt, Cursor, Claude Code or whatever built the app. Replace anything in capitals. It never asks you to paste a secret into a chat; keys go into your host’s environment, by you.
Error capture shows “Cannot read properties of undefined (reading map)” on /YOUR-PAGE since my last publish. Find where that page maps over data, make the component handle an empty or missing list without throwing, and check the data loader returns an array in every branch. Show me the diff before applying it.
How Keelnest catches it
Error capture counts each error against its usual rate and opens an incident on a spike, with the time it began. When the publish before it is already under suspicion, the spike joins that incident as evidence, and the publish's page offers the last verified revision to roll back to.
Know the moment this happens.
Keelnest watches your production app every five minutes and tells you in plain English what broke — with the fix. Free for your first app.