add
Adds a new style source. Note: When adding a geojson source, this method does not synchronously parse the GeoJSON data. The events API shall be used to make sure the provided GeoJSON data is valid. In case the GeoJSON is valid, a MapLoaded event will be propagated. In case of errors, a MapLoadingError event will be propagated instead.
Return
A string describing an error if the operation was not successful, empty otherwise.
Parameters
source
An identifier for the style source.
properties
A map of style source properties.