In that case, you dont need to add to build.json, You will have to check whether the erpnext.taxes_and_totals class is available before extending it. Try debugging it using the debugger statement in JS
In that case, you dont need to add to build.json, You will have to check whether the erpnext.taxes_and_totals class is available before extending it. Try debugging it using the debugger statement in JS