How to create an array runtime field

Elastic Stack Kibana
10

I have tried using param._source as suggested, but still not working.

This is not populating the runtime field:

PUT employee/_mappings
{
  "runtime": {
    "empRoles": {
      "type": "keyword",
      "script": """if (doc["roles.name.raw"].size()!=0 ) {
        for(item in params._source.roles.name.raw) {
          emit(item) ;
        }
        
         }
         """
    }
  }
}

The following one results in error cannot cast from [java.lang.String[]] to [void], as expected:

PUT employee/_mappings
{
  "runtime": {
    "empRoles": {
      "type": "keyword",
      "script": """String[] empRoles;
        if (doc["roles.name.raw"].size()!=0 ) {
        int i=0;
        for(item in params._source.roles.name.raw) {
          empRoles[i++]=item
        }
        
         }
return empRoles;
         """
    }
  }
}