Logs you can debug from

A log line is a message to someone debugging production at the worst possible moment, possibly you. console.log("here", user) doesn't help them. What helps: one event per line, as JSON, with the same fields every time: a timestamp, a level, a short fixed message, and the details as named fields. Log tools can then filter (level=error), group (message="payment failed") and count, instead of someone grepping prose.

{"time":"2026-10-09T09:14:02.118Z","level":"error","message":"payment failed","requestId":"r-8f2c","userId":"u_381","provider":"stripe","status":402,"durationMs":812}

Give every request an id and put it on every line that request writes, and in the 500 response the user sees. Then a support ticket that says "error r-8f2c" leads straight to the full story. Keep the message fixed ("payment failed") and put the variable parts in fields. A message like "payment of 500 failed for u_381" is unique on every line, so you can never count how often it happens.

Use levels honestly. error: something failed and someone may need to act. warn: unexpected, but handled. info: normal events worth keeping, such as a request finished or a job ran. debug: detail you switch on while investigating. An app that logs every request at error has trained everyone to ignore errors.

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