type guard function that redefines a TypeLegend as a TypeWmsLegend if the event attribute of the verifyIfPayload parameter is valid. The type ascention applies only to the true block of the if clause.
object to test in order to determine if the type ascention is valid
returns true if the payload is valid
type guard function that redefines a TypeLegend as a TypeWmsLegend if the event attribute of the verifyIfPayload parameter is valid. The type ascention applies only to the true block of the if clause.