Sample Script to fetch expiry_date field from Batch doctype to Sales Invoice Item table

Step 1: Create Custom Script for Sales Invoice (parent) doctype

Step 2: Script as below & Save

frappe.ui.form.on("Sales Invoice Item", "batch_no", function(frm, cdt, cdn) {

  var d = locals[cdt][cdn];

    frappe.db.get_value("Batch", {"name": d.batch_no}, "expiry_date", function(value) {

      d.expiry_date = value.expiry_date;

    });

});