You can view an existing calculated field the see the JavaScript expression that generates calculated values.
About this Task
When you view a calculated field in a mapping rule:
The field name is shown in italics.
The field does not have an include/exclude checkbox.
The field does not state its data type.
Steps
From the Mapping screen, click a table or collection name on the Schema model pane or diagram view.
Click the pencil icon next to the mapping rule you want to view.
Hover over the i icon next to the calculated field you want to view. The JavaScript expression displays in a tooltip window.