Add unlisted values automatically to unique value symbology

[RehanChaudhary]

i have a webmap set up in mapviewer that comes with arcgis enterprise portal . the webmap uses unique values symbology for the feature layer added to it . The feature layer gets updated with new data from time to time but whenever there is new data, all the new values without symbology go into "unlisted values".

How can i automatically assign symbologies or colors to new data thats coming which previously has no colors? 

[DanPatterson]

UniqueValueRenderer—ArcGIS Pro | Documentation

Once you apply your fields, the renderer will automatically generate all the unique values. If, for whatever reason, new values are added to the layer, you will need to reset the renderer so that all the values are added again. You can do this by changing the fields property, or you can use the addValues method.

but you will need to get a list of new values using

listMissingValues ()

It is probably easier to rebuild it from scratch using the field