# Transformers

Transformers allow to replace the values of specified keys within payloads on-demand. Oddlog uses a few helpful transformers by default: fn, err, req, res (see documentation).

Transformers can be passed as option to createLogger/defineLogger. The standard transformers are exported as oddlog.stdTransformer; this way one can use them in addition to custom ones:

oddlog
  .createLogger("my-app", {
    transformer: oddlog.assign(
      { hex: (x) => x.toString(16).toUpperCase() },
      oddlog.stdTransformer
    )
  })
  .info({hex: 255}); // logs with payload `{"hex": "FF"}`