During October 2025, Els Bree enhanced the DefinitelyTyped/DefinitelyTyped repository by adding custom map ID support to Google Mutant layers in Leaflet.gridlayer.googlemutant. They introduced a mapId parameter to GoogleMutantOptions, enabling developers to specify a custom Google Maps ID for tailored map rendering. The implementation, written in TypeScript and leveraging mapping libraries such as Leaflet, included a dedicated test case to demonstrate the new feature’s usage and ensure typing accuracy. Els Bree’s contribution improved the flexibility of map rendering options for users and increased test coverage, reflecting a focused and well-scoped engineering effort within the project’s type definitions.