TypeScript doesn't resolve the correct type of the logger inside controller (`req.log`)

[svex99]

Problem

I have a logger with a custom level that is used as base to create the logger passed to Express.

import pino from 'pino';
import pinoHTTP from "pino-http";

export const logger = pino<"newLevel">({
    customLevels: {
        newLevel: 100,
    },
})

const httpLogger = pinoHTTP({
    logger: logger,
});

export default httpLogger;

However when I access the log property from the request it doesn't point to the right type of the logger being used (Logger<"newLevel">), instead it points to Logger<never> (from the base pino.d.ts AFAIK).

This causes TypeScript to complain when called the method req.log.newLevel(...) inside a controller.

app.get('/test', (req, res) => {
    req.log.trace('trace log') // TypeScript doesn't complain!
    req.log.newLevel('newLevel log')
    //      \__ Property 'newLevel' does not exist on type 'Logger<never>'.ts(2339)
    res.status(200)
})

Question

How could I point TypeScript to look at the correct type of the logger inside the controllers when using custom levels?

Thanks in advance!

[mahmoodhamdi]

The issue is that pino-http's type definitions don't propagate custom level generics to req.log. The HttpLogger type uses Logger<never> for the augmented request.

Workaround using module augmentation:

import pino from 'pino'
import pinoHTTP from 'pino-http'

type CustomLevels = 'newLevel'

export const logger = pino<CustomLevels>({
  customLevels: { newLevel: 100 }
})

// Augment the type to include custom levels
declare module 'http' {
  interface IncomingMessage {
    log: pino.Logger<CustomLevels>
  }
}

const httpLogger = pinoHTTP({ logger })

// Now in your controller:
app.get('/test', (req, res) => {
  req.log.newLevel('works!')  // No TS error
  req.log.trace('also works!')
  res.status(200).send('ok')
})

Alternatively, you can use a type assertion at the usage site:

app.get('/test', (req, res) => {
  const log = req.log as pino.Logger<'newLevel'>
  log.newLevel('newLevel log')
})

The proper fix would be to make HttpLogger generic over custom levels so req.log inherits the type from the logger option, but that would require a change to pino-http's type definitions.