Esri Basemaps
You can find the list of available basemap string identifiers for the ArcGIS Maps SDK for JavaScript in the official Esri documentation. This includes both 2D and 3D basemap IDs.
3D Basemaps (for use with SceneView)
topo-3d– Topographicnavigation-3d– Navigationnavigation-dark-3d– Navigation (Dark)osm-3d– OpenStreetMapgray-3d– Light Gray Canvasdark-gray-3d– Dark Gray Canvasstreets-3d– Streetsstreets-dark-3d– Streets (Dark)
2D Basemaps (for use with MapView)
topo/topo-vector– Topographicstreets/streets-vector– Streetsstreets-night-vector– Streets (Night)streets-navigation-vector– Navigationstreets-relief-vector– Streets with Reliefgray/gray-vector– Light Gray Canvasdark-gray/dark-gray-vector– Dark Gray Canvassatellite– World Imageryhybrid– Imagery with Labelsoceans– Ocean Basemapterrain– Terrain with Labelsosm– OpenStreetMap
For the most up-to-date and comprehensive list, refer to the Map class API reference.