How to fetch data in child table new row only?


I have created a custom field Fixed Credit Limit and a child table Credit Limit History which has a column named fixed_limit under customer doctype.

Now I have created a custom script which fetches the value from parent doctype Fixed Credit Limit in every row in the child table. Now, If I change the value in parent field, then every row value gets changed. How do I prevent changing the previously fetched values in the rows? and make this script work for only the new rows?

The current script is like this:

function table_calc(frm) {
    for ( let i in frm.doc.customer_credit_history){
        frm.doc.customer_credit_history[i].fixed_limit = frm.doc.fixed_credit_limit;

frappe.ui.form.on("Customer Credit History", "customer_credit_history_add", table_calc);