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#4960·fastify

TypeScript issue when passing Pino instance to fastify constructor

Author: pavelkornevCreated Aug 8, 2023Updated Sep 17, 2026
Labelsbugtypescript

Prerequisites

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

Fastify version

4.21.0

Plugin version

No response

Node.js version

18.14.2

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

13.5

Description

By some historical reasons we pass a Pino instance to fastify factory function (I'm aware that Pino is built-in into fastify):

const logger = pino({
  // ...some config
});

const server = fastify({
  logger,
  // ...other opts
});

But starting v4.20.0 this code is no longer working with the following TypeScript compile error:

Type 'FastifyInstance<RawServerDefault, IncomingMessage, ServerResponse<IncomingMessage>, Logger<LoggerOptions | DestinationStream>, FastifyTypeProviderDefault>' is not assignable to type 'FastifyInstance<RawServerDefault, IncomingMessage, ServerResponse<IncomingMessage>, FastifyBaseLogger, FastifyTypeProviderDefault>'.
  The types of 'withTypeProvider().childLoggerFactory' are incompatible between these types.
    Type 'FastifyChildLoggerFactory<RawServerDefault, IncomingMessage, ServerResponse<IncomingMessage>, Logger<LoggerOptions | DestinationStream>, any>' is not assignable to type 'FastifyChildLoggerFactory<RawServerDefault, IncomingMessage, ServerResponse<IncomingMessage>, FastifyBaseLogger, any>'.
      The 'this' types of each signature are incompatible.
        Type 'FastifyInstance<RawServerDefault, IncomingMessage, ServerResponse<IncomingMessage>, FastifyBaseLogger, any>' is not assignable to type 'FastifyInstance<RawServerDefault, IncomingMessage, ServerResponse<IncomingMessage>, Logger<LoggerOptions | DestinationStream>, any>'.
          Types of property 'log' are incompatible.
            Type 'FastifyBaseLogger' is not assignable to type 'Logger<LoggerOptions | DestinationStream>

It seems to be related to this recent change - https://github.com/fastify/fastify/pull/4760

Steps to Reproduce

  1. Create Pino instance
  2. Try to pass to fastify factory function

Expected Behavior

No response

Source: fastify/fastify

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