I am trying to create a DA in order to show as an alert the value of my item page P2003_TEMP_DOUBLE. This value is not stored to any table in my database. The result of this action is a null alert!
Following the javascript api of oracle apex you must apply the bind variables in this way
alert( "P1_ITEM value = " + apex.item( "P1_ITEM" ).getValue() );
Ref. https://docs.oracle.com/database/121/AEAPI/javascript_api.htm#AEAPI29437