Make Read Only After Saving

Use the method cur_frm.set_df_property to update the field's display.

In this script we also use the __islocal property of the DocType to check if the document has been saved at least once or is never saved. If __islocal is 1, then the document has never been saved.

frappe.ui.form.on("MyDocType", "refresh", function(frm) {
 // use the is_new method of frm, to check if the doc is saved or not
 frm.set_df_property("myfield", "read_only", frm.is_new() ? 0 : 1);
}