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Last active June 12, 2025 11:44
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Logging in Fastify
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logging_in_fastify.md

TLDR:

  • If you want to log an object, put the object as the first argument, and (optionally) a message string as the second argument.
  • If you want to log an error object, it must be the first argument, and not a property inside an object.
  • (As a result, I don't know how to log an error and also an object for context, but it might be possible.)

Patterns which are WORKING:

fastify.log.info("message")
fastify.log.info(error, "message")
fastify.log.info({ data }, "message")

Patterns which DO NOT WORK:

// Only logs the message, ignores the data object!
fastify.log.info("message", { data })

// Only logs: { error: "[object Error]" }
fastify.log.info({ error }, "message")
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