Finding faults.
WHERE TO LOOK FIRST
The browser console, for frontend errors The server log, for backend errors The network panel, for request failures
WHAT AN ERROR USUALLY STATES
The file, the line and the cause.
WHAT TO DO
Read it, literally, before changing anything.
WHAT TO ESTABLISH
Whether it is frontend or backend Whether it is reproducible What changed beforehand
HOW TO DISTINGUISH FRONTEND FROM BACKEND
Look at the network panel. If the response is wrong, it is the backend. If the response is correct and the page is wrong, it is the frontend.
WHAT THAT SAVES
Considerable time spent in the wrong place.
WHAT TO USE FOR INSPECTING VALUES
The debugger, which pauses and shows everything Console output, for quick checks
WHAT TO PRINT
The value and its type.
WHY
A surprising number of bugs are a value being a different type than assumed.
WHAT TO DO WHEN IT WORKS LOCALLY AND FAILS DEPLOYED
Check versions, environment values, caches, permissions and filename case.
WHAT TO CHANGE
One thing at a time.
