Version 1.0
temp = thisapp.udf_history.get_field_changes(input.HIDDEN_FIELD_CHANGES, "POSTCODE", ps.POSTCODE.toString());
However, when I made changes to my script, I kept getting errors so I was forced to use
Version 1.3
postcode_string = ps.POSTCODE.toString();
temp = thisapp.udf_history.get_field_changes(input.HIDDEN_FIELD_CHANGES, "POSTCODE",postcode_string);
I think zoho creator has issues with nested functions.