Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

It is possible to pass an existing logger object to the fastify() function. There is a check here that detects if the logger instance is valid.

If a partial logger is passed in, it is currently ignored because that isValidLogger() function returns false. If I pass in an incorrect logger object, it would be more helpful to know that it was rejected rather than silently ignored. I propose throwing if a partially matching logger is provided.

Motivation

I spent a few hours debugging why my logger was not being used. It turns out that pino's child() function is not enumerable, and @fastify/deepmerge only clones enumerable properties. I created a fastify server with the cloned logger, and it was silently ignored because child() was missing.

Example

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