I think, in js, it’s hard to implement for a child to parent fieldset.
Please apply the server script.
Script:
for d in doc.get('items'):
batch_name = frappe.db.get_value("Batch", {'name': d.batch_no, 'item': d.item_code}, 'name')
if batch_name:
bn = frappe.get_doc("Batch", batch_name)
bn.crop_type = d.crop_type
bn.save()
Thank You!