brotherton-erpnext/erpnext/docs/user/customize-erpnext/custom-scripts/custom-script-examples/sales-invoice-id-based-on-sales-order-id.md

Below script allows you to get naming series in Sales Invoice, same as of corresponding Sales Order. Invoice uses a prefix M- but the number duplicates the SO doc name (number).

Example: If Sales Order id is SO-12345, then corresponding Sales Invoice id will be set as M-12345.

frappe.ui.form.on("Sales Invoice", "refresh", function(frm){
    var sales_order = frm.doc.items[0].sales_order.replace("M", "M-");
    if (!frm.doc.__islocal && sales_order && frm.doc.name!==sales_order){
        frappe.call({
        method: 'frappe.model.rename_doc.rename_doc',
        args: {
            doctype: frm.doctype,
            old: frm.docname,
            "new": sales_order,
            "merge": false
       },
    });
    }
});