Function existsById

existsById(layerId, thematicId): boolean
JMap.Layer.Thematic.existsById

Returns true if the thematic exists for the given layer and thematic ids, false otherwise.
Parameters
- layerId: JId
  The JMap layer id
- thematicId: JId
  The thematic id
Returns boolean
Example
```
// test for thematic existence
JMap.Layer.Thematic.existsById(4, 3)
// false
```