HTML,js client: Get changing item name

[ANYELEC]

The callback called by forEachChangedField receives the field information in its arguments.
Do you mean that the "fieldName" argument is null?
This means that the field names were not supplied by the application in the subscription specification.
In other words, the setFieldSchema method was used on the Subscription object, to supply a "schema name" to be mapped on a set of fields on the server side.
In this case, only the numeric "fieldPos" argument is supplied and it is assumed that the client application also knows the mapping.
Do you see anything different?

OK. Following your suggestion now it works.