How to use object.assign() method

function invoiceLineColumn(invId) {
            try {
                let invoiceSearchObj = search.create({
                    type: "invoice",
                    filters:
                        [
                            ["type", "anyof", "CustInvc"],
                            "AND",
                            ["mainline", "is", "F"],
                            "AND",
                            ["internalid", "anyof", invId],
                            "AND",
                            ["shipping", "is", "F"],
                            "AND",
                            ["taxline", "is", "F"],
                            "AND",
                            ["cogs", "is", "F"]
                        ],
                    columns:
                        [
                            search.createColumn({ name: "item", label: "Item" }),
                            search.createColumn({ name: "line", label: "Line ID" }),
                            search.createColumn({ name: "quantity", label: "Quantity" }),
                            search.createColumn({ name: "datecreated", label: "Date Created" }),
                            search.createColumn({ name: "type", label: "Type" }),
                            search.createColumn({ name: "custcol_jj_lineid_invoice", label: "lineId_invoice" })
                        ]
                });
                let searchResultCount = invoiceSearchObj.runPaged().count;
                let extObj = {};

                if (searchResultCount > 0) {

                    let searchResult = invoiceSearchObj.run();
                    searchResult.each(function (result) {
                        let intObj = {};
                        extObj[result.getValue(searchResult.columns[5])] = { "lineId": result.getValue(searchResult.columns[1]), "itemName": result.getText(searchResult.columns[0]), "itemQty": result.getValue(searchResult.columns[2]), "createddate": result.getValue(searchResult.columns[3]), "typeTr": result.getValue(searchResult.columns[4]) };
                        Object.assign(extObj, intObj)
                        return true;
                    });
                    console.log("object", extObj);
                    return extObj;
                }
                else {
                    return {};
                }
            }
            catch (err) {
                console.error("error@invoiceLineColumn", err);
            }
        }