I used code to hide certain fields in my custom invoice form as below.

/**

 * @NApiVersion 2.1

 * @NScriptType UserEventScript

 */

 define(['N/runtime','N/record','N/ui/serverWidget'],

    function (runtime,record,serverWidget) {

        var exports = {};

        function beforeLoad(scriptContext) {

            var form = scriptContext.form

            var user = runtime.getCurrentUser()

            //MAIN FIELDS

            //Hide country of origin

            var countryoforigin = form.getField({id: 'custpage_country_of_origin_temp'})

            countryoforigin.updateDisplayType({displayType: serverWidget.FieldDisplayType.HIDDEN})

            //Hide counterparty vat number

            var counterpartyvatnumber = form.getField({id: 'custbody_counterparty_vat'})

            counterpartyvatnumber.updateDisplayType({displayType: serverWidget.FieldDisplayType.HIDDEN})

            //Hide delivery terms

            var deliveryterms = form.getField({id: 'custpage_delivery_terms'})

            deliveryterms.updateDisplayType({displayType: serverWidget.FieldDisplayType.HIDDEN})

            //Hide nature of transaction code