JSON to CSV flattening nested JSON

If you are looking for the fix to your actual code then you need to use something like below

function flatten(object, addToList, prefix) {    Object.keys(object).map(key => {        if (object[key] === null) {            addToList[prefix + key] = "";        } else        if (object[key] instanceof Array) {            // addToList[prefix + key] = listToFlatString(object[key]);            for (i in object[key]) {                flatten(object[key][i], addToList, prefix + key + "." + i)            }        } else if (typeof object[key] == 'object' && !object[key].toLocaleDateString) {            flatten(object[key], addToList, prefix + key + '.');        } else {            addToList[prefix + key] = object[key];        }    });    return addToList;}

I replace

addToList[prefix + key] = listToFlatString(object[key]);

with

for (i in object[key]) {   flatten(object[key][i], addToList, prefix + key + "." + i)}

That fixed the issue and the output is below as per the sample json you provided

[ '0.countyCode': 12,  '0.excludedFlag': '',  '0.fees.annualTotalFees': 35.6,  '0.fees.annualTotalFeesMail': 36.35,  '0.fees.annualTotalFeesOnline': 38.35,  '0.fees.biennialTotalFees': 71.2,  '0.fees.biennialTotalFeesMail': 71.95,  '0.fees.biennialTotalFeesOnline': 73.95,  '0.fees.branchFeeFlag': false,  '0.fees.delinquentFeeAmount': 5,  '0.fees.mhBackTax': 0,  '0.fees.mhBackTaxMonths': 0,  '0.fileId': 522,  '0.id': '0002be04-546-4a92-a3d7-c31546544f',  '0.messages.0messageCode': 'RN2',  '0.messages.0messageText': 'Plate must be replaced if a 2 year renewal is desired.',  '0.messages.0messageType': 'I',  '0.messages.1messageCode': 'RN40',  '0.messages.1messageText': 'You must complete the affidavit on the reverse side or provide a copy of your Florida insurance identification card.',  '0.messages.1messageType': 'I',  '0.messages.2messageCode': 'RN41',  '0.messages.2messageText': 'Insurance is on file.',  '0.messages.2messageType': 'II',  '0.registrationDetail.annualPlateReplacementFlag': false,  '0.registrationDetail.arfCredit': 25.6,  '0.registrationDetail.biennialPlateReplacementFlag': true,  '0.registrationDetail.customerStopFlag': '',  '0.registrationDetail.delinquentDate': '2018-02-11T00:00:00',  '0.registrationDetail.expirationDate': '2018-01-09T00:00:00',  '0.registrationDetail.foreignAddressFlag': false,  '0.registrationDetail.honorayConsulPlateFlag': '',  '0.registrationDetail.hovDecalNumber': '',  '0.registrationDetail.hovDecalYear': '',  '0.registrationDetail.hovExpirationDate': '',  '0.registrationDetail.inventorySubtype': 'RP',  '0.registrationDetail.lastActivityCounty': 12,  '0.registrationDetail.legislativePlateDueForReplacementFlag': false,  '0.registrationDetail.licensePlateCode': 'RGR',  '0.registrationDetail.licensePlateNumber': 'L45656',  '0.registrationDetail.mailToAddressFlag': false,  '0.registrationDetail.mailToCustomerNumber': '',  '0.registrationDetail.mhLocationCode': '',  '0.registrationDetail.militaryOwnerFlag': false,  '0.registrationDetail.numberOfRegistrants': 1,  '0.registrationDetail.onlineRenewalEligibilityFlag': true,  '0.registrationDetail.pinNumber': 222354654,  '0.registrationDetail.plateExpirationDate': '2018-03-18T00:00:00',  '0.registrationDetail.plateIssueDate': '2008-09-18T00:00:00',  '0.registrationDetail.possibleNgExemptionFlag': false,  '0.registrationDetail.registrationNumber': 2234545345,  '0.registrationDetail.registrationOnlyFlag': '',  '0.registrationDetail.registrationType': 'R',  '0.registrationDetail.registrationUse': 'PR',  '0.registrationDetail.renewalCountyCode': 12,  '0.registrationDetail.rentalParkFlag': '',  '0.registrationDetail.seminoleMiccosukeeIndianFlag': false,  '0.registrationDetail.taxCollectorRenewalEligibilityFlag': true,  '0.registrationDetail.vehicleClassCode': 1,  '0.registrationOwners.0customer.addresses.0addressType': 'R',  '0.registrationOwners.0customer.addresses.0city': 'PALMETTO',  '0.registrationOwners.0customer.addresses.0countryCode': 'US',  '0.registrationOwners.0customer.addresses.0countyCode': 12,  '0.registrationOwners.0customer.addresses.0foreignPostalCode': '',  '0.registrationOwners.0customer.addresses.0state': 'FL',  '0.registrationOwners.0customer.addresses.0streetAddress': '34545 7TH AVE W',  '0.registrationOwners.0customer.addresses.0zipCode': 34221,  '0.registrationOwners.0customer.addresses.0zipPlus': '',  '0.registrationOwners.0customer.addresses.1addressType': 'M',  '0.registrationOwners.0customer.addresses.1city': 'PALMETTO',  '0.registrationOwners.0customer.addresses.1countryCode': 'US',  '0.registrationOwners.0customer.addresses.1countyCode': 12,  '0.registrationOwners.0customer.addresses.1foreignPostalCode': '',  '0.registrationOwners.0customer.addresses.1state': 'FL',  '0.registrationOwners.0customer.addresses.1streetAddress': '34545 7TH AVE W',  '0.registrationOwners.0customer.addresses.1zipCode': 34221,  '0.registrationOwners.0customer.addresses.1zipPlus': 3433,  '0.registrationOwners.0customer.companyName': '',  '0.registrationOwners.0customer.customerNumber': 3346645,  '0.registrationOwners.0customer.customerStopFlag': false,  '0.registrationOwners.0customer.customerType': 'I',  '0.registrationOwners.0customer.dateOfBirth': '1971-01-09T00:00:00',  '0.registrationOwners.0customer.dlExpirationDate': '2025-01-09T00:00:00',  '0.registrationOwners.0customer.dlRenewalEligibilityFlag': true,  '0.registrationOwners.0customer.driverLicenseNumber': 'F34545345345',  '0.registrationOwners.0customer.emailAddress': '',  '0.registrationOwners.0customer.feidNumber': '',  '0.registrationOwners.0customer.firstName': 'DAVID',  '0.registrationOwners.0customer.lastName': 'HUGH',  '0.registrationOwners.0customer.middleName': 'BRIERTON',  '0.registrationOwners.0customer.militaryExemptionFlag': false,  '0.registrationOwners.0customer.nameSuffix': '',  '0.registrationOwners.0customer.sex': 'F',  '0.registrationOwners.0registrationOwnershipNumber': 1,  '0.shippingAddress.address.addressType': 'S',  '0.shippingAddress.address.city': 'PALMETTO',  '0.shippingAddress.address.countryCode': 'US',  '0.shippingAddress.address.countyCode': 12,  '0.shippingAddress.address.foreignPostalCode': '',  '0.shippingAddress.address.state': 'FL',  '0.shippingAddress.address.streetAddress': '34545 7TH AVE W',  '0.shippingAddress.address.zipCode': 34221,  '0.shippingAddress.address.zipPlus': 8344,  '0.shippingAddress.shippingCompanyName': '',  '0.shippingAddress.shippingName1': 'DAVID HUGH BRIERTON',  '0.shippingAddress.shippingName2': '',  '0.stops': '',  '0.vehicle.address': '',  '0.vehicle.bodyCode': '4D',  '0.vehicle.brakeHorsePower': '',  '0.vehicle.cubicCentimeters': '',  '0.vehicle.grossWeight': '',  '0.vehicle.insuranceAffidavitCode': '',  '0.vehicle.leaseOwnerFlag': false,  '0.vehicle.lengthFeet': '',  '0.vehicle.lengthInches': '',  '0.vehicle.majorColorCode': 'GLD',  '0.vehicle.makeCode': 'CHEV',  '0.vehicle.minorColorCode': '',  '0.vehicle.netWeight': 3200,  '0.vehicle.numberOfAxles': '',  '0.vehicle.ownerUnitNumber': '',  '0.vehicle.titleNumber': 345454534,  '0.vehicle.vehicleIdentificationNumber': '1G1ZD345U5B345435',  '0.vehicle.vehicleNumber': 23454656,  '0.vehicle.vehicleType': 'AU',  '0.vehicle.vesselCode': '',  '0.vehicle.vesselManufacturerDesc': '',  '0.vehicle.vesselResidentStatus': 'Y',  '0.vehicle.vesselWaterType': '',  '0.vehicle.widthFeet': '',  '0.vehicle.widthInches': '',  '0.vehicle.yearMake': 2011,  '1.countyCode': 12,  '1.excludedFlag': '',  '1.fees.annualTotalFees': 27.6,  '1.fees.annualTotalFeesMail': 28.35,  '1.fees.annualTotalFeesOnline': 30.35,  '1.fees.biennialTotalFees': 55.2,  '1.fees.biennialTotalFeesMail': 55.95,  '1.fees.biennialTotalFeesOnline': 57.95,  '1.fees.branchFeeFlag': false,  '1.fees.delinquentFeeAmount': 5,  '1.fees.mhBackTax': 0,  '1.fees.mhBackTaxMonths': 0,  '1.fileId': 522,  '1.id': '0008c654-8960-45b8-b416-cff3456767',  '1.messages.0messageCode': 'RN40',  '1.messages.0messageText': 'You must complete the affidavit on the reverse side or provide a copy of your Florida insurance identification card.',  '1.messages.0messageType': 'I',  '1.registrationDetail.annualPlateReplacementFlag': false,  '1.registrationDetail.arfCredit': 2.8,  '1.registrationDetail.biennialPlateReplacementFlag': false,  '1.registrationDetail.customerStopFlag': '',  '1.registrationDetail.delinquentDate': '2018-02-11T00:00:00',  '1.registrationDetail.expirationDate': '2018-01-01T00:00:00',  '1.registrationDetail.foreignAddressFlag': false,  '1.registrationDetail.honorayConsulPlateFlag': '',  '1.registrationDetail.hovDecalNumber': '',  '1.registrationDetail.hovDecalYear': '',  '1.registrationDetail.hovExpirationDate': '',  '1.registrationDetail.inventorySubtype': 'SS',  '1.registrationDetail.lastActivityCounty': 16,  '1.registrationDetail.legislativePlateDueForReplacementFlag': false,  '1.registrationDetail.licensePlateCode': 'RGS',  '1.registrationDetail.licensePlateNumber': 'HU34598',  '1.registrationDetail.mailToAddressFlag': false,  '1.registrationDetail.mailToCustomerNumber': '',  '1.registrationDetail.mhLocationCode': '',  '1.registrationDetail.militaryOwnerFlag': false,  '1.registrationDetail.numberOfRegistrants': 1,  '1.registrationDetail.onlineRenewalEligibilityFlag': true,  '1.registrationDetail.pinNumber': 4936856,  '1.registrationDetail.plateExpirationDate': '2026-09-24T00:00:00',  '1.registrationDetail.plateIssueDate': '2017-03-24T00:00:00',  '1.registrationDetail.possibleNgExemptionFlag': false,  '1.registrationDetail.registrationNumber': 4095685,  '1.registrationDetail.registrationOnlyFlag': '',  '1.registrationDetail.registrationType': 'R',  '1.registrationDetail.registrationUse': 'PR',  '1.registrationDetail.renewalCountyCode': 12,  '1.registrationDetail.rentalParkFlag': '',  '1.registrationDetail.seminoleMiccosukeeIndianFlag': false,  '1.registrationDetail.taxCollectorRenewalEligibilityFlag': true,  '1.registrationDetail.vehicleClassCode': 1,  '1.registrationOwners.0customer.addresses.0addressType': 'R',  '1.registrationOwners.0customer.addresses.0city': 'SARASOTA',  '1.registrationOwners.0customer.addresses.0countryCode': 'US',  '1.registrationOwners.0customer.addresses.0countyCode': 12,  '1.registrationOwners.0customer.addresses.0foreignPostalCode': '',  '1.registrationOwners.0customer.addresses.0state': 'FL',  '1.registrationOwners.0customer.addresses.0streetAddress': '5858 FRUITVILLE RD',  '1.registrationOwners.0customer.addresses.0zipCode': 34240,  '1.registrationOwners.0customer.addresses.0zipPlus': 5858,  '1.registrationOwners.0customer.addresses.1addressType': 'M',  '1.registrationOwners.0customer.addresses.1city': 'SARASOTA',  '1.registrationOwners.0customer.addresses.1countryCode': 'US',  '1.registrationOwners.0customer.addresses.1countyCode': 16,  '1.registrationOwners.0customer.addresses.1foreignPostalCode': '',  '1.registrationOwners.0customer.addresses.1state': 'FL',  '1.registrationOwners.0customer.addresses.1streetAddress': '5858 FRUITVILLE RD',  '1.registrationOwners.0customer.addresses.1zipCode': 34240,  '1.registrationOwners.0customer.addresses.1zipPlus': 5858,  '1.registrationOwners.0customer.companyName': '',  '1.registrationOwners.0customer.customerNumber': 2928357,  '1.registrationOwners.0customer.customerStopFlag': false,  '1.registrationOwners.0customer.customerType': 'I',  '1.registrationOwners.0customer.dateOfBirth': '1989-01-01T00:00:00',  '1.registrationOwners.0customer.dlExpirationDate': '2022-01-01T00:00:00',  '1.registrationOwners.0customer.dlRenewalEligibilityFlag': true,  '1.registrationOwners.0customer.driverLicenseNumber': 'B94832734',  '1.registrationOwners.0customer.emailAddress': '',  '1.registrationOwners.0customer.feidNumber': '',  '1.registrationOwners.0customer.firstName': 'DAVID1',  '1.registrationOwners.0customer.lastName': 'HUGH1',  '1.registrationOwners.0customer.middleName': 'BRIERTON1',  '1.registrationOwners.0customer.militaryExemptionFlag': false,  '1.registrationOwners.0customer.nameSuffix': '',  '1.registrationOwners.0customer.sex': 'M',  '1.registrationOwners.0registrationOwnershipNumber': 1,  '1.shippingAddress.address.addressType': 'S',  '1.shippingAddress.address.city': 'SARASOTA',  '1.shippingAddress.address.countryCode': 'US',  '1.shippingAddress.address.countyCode': 16,  '1.shippingAddress.address.foreignPostalCode': '',  '1.shippingAddress.address.state': 'FL',  '1.shippingAddress.address.streetAddress': '5858 FRUITVILLE RD',  '1.shippingAddress.address.zipCode': 34240,  '1.shippingAddress.address.zipPlus': 5858,  '1.shippingAddress.shippingCompanyName': '',  '1.shippingAddress.shippingName1': 'DAVID1 HUGH1 BRIERTON1',  '1.shippingAddress.shippingName2': '',  '1.stops': '',  '1.vehicle.address': '',  '1.vehicle.bodyCode': '4D',  '1.vehicle.brakeHorsePower': '',  '1.vehicle.cubicCentimeters': '',  '1.vehicle.grossWeight': '',  '1.vehicle.insuranceAffidavitCode': '',  '1.vehicle.leaseOwnerFlag': false,  '1.vehicle.lengthFeet': '',  '1.vehicle.lengthInches': '',  '1.vehicle.majorColorCode': 'BLU',  '1.vehicle.makeCode': 'STRN',  '1.vehicle.minorColorCode': '',  '1.vehicle.netWeight': 2290,  '1.vehicle.numberOfAxles': '',  '1.vehicle.ownerUnitNumber': '',  '1.vehicle.titleNumber': 239874,  '1.vehicle.vehicleIdentificationNumber': '1G832492871Z23094',  '1.vehicle.vehicleNumber': 239084,  '1.vehicle.vehicleType': 'AU',  '1.vehicle.vesselCode': '',  '1.vehicle.vesselManufacturerDesc': '',  '1.vehicle.vesselResidentStatus': 'Y',  '1.vehicle.vesselWaterType': '',  '1.vehicle.widthFeet': '',  '1.vehicle.widthInches': '',  '1.vehicle.yearMake': 2001,  '2.countyCode': 12,  '2.excludedFlag': '',  '2.fees.annualTotalFees': 45.6,  '2.fees.annualTotalFeesMail': 46.35,  '2.fees.annualTotalFeesOnline': 48.35,  '2.fees.biennialTotalFees': 91.2,  '2.fees.biennialTotalFeesMail': 91.95,  '2.fees.biennialTotalFeesOnline': 93.95,  '2.fees.branchFeeFlag': false,  '2.fees.delinquentFeeAmount': 10,  '2.fees.mhBackTax': 0,  '2.fees.mhBackTaxMonths': 0,  '2.fileId': 522,  '2.id': '000e3450d-3454-499a-ae70-de5676577',  '2.messages.0messageCode': 'RN40',  '2.messages.0messageText': 'You must complete the affidavit on the reverse side or provide a copy of your Florida insurance identification card.',  '2.messages.0messageType': 'I',  '2.registrationDetail.annualPlateReplacementFlag': false,  '2.registrationDetail.arfCredit': 8.4,  '2.registrationDetail.biennialPlateReplacementFlag': false,  '2.registrationDetail.customerStopFlag': '',  '2.registrationDetail.delinquentDate': '2018-02-11T00:00:00',  '2.registrationDetail.expirationDate': '2018-01-11T00:00:00',  '2.registrationDetail.foreignAddressFlag': false,  '2.registrationDetail.honorayConsulPlateFlag': '',  '2.registrationDetail.hovDecalNumber': '',  '2.registrationDetail.hovDecalYear': '',  '2.registrationDetail.hovExpirationDate': '',  '2.registrationDetail.inventorySubtype': 'RP',  '2.registrationDetail.lastActivityCounty': 12,  '2.registrationDetail.legislativePlateDueForReplacementFlag': false,  '2.registrationDetail.licensePlateCode': 'RGR',  '2.registrationDetail.licensePlateNumber': '808IUT',  '2.registrationDetail.mailToAddressFlag': false,  '2.registrationDetail.mailToCustomerNumber': '',  '2.registrationDetail.mhLocationCode': '',  '2.registrationDetail.militaryOwnerFlag': false,  '2.registrationDetail.numberOfRegistrants': 1,  '2.registrationDetail.onlineRenewalEligibilityFlag': true,  '2.registrationDetail.pinNumber': 934597,  '2.registrationDetail.plateExpirationDate': '2023-06-06T00:00:00',  '2.registrationDetail.plateIssueDate': '2013-12-06T00:00:00',  '2.registrationDetail.possibleNgExemptionFlag': false,  '2.registrationDetail.registrationNumber': 39287432,  '2.registrationDetail.registrationOnlyFlag': '',  '2.registrationDetail.registrationType': 'R',  '2.registrationDetail.registrationUse': 'PR',  '2.registrationDetail.renewalCountyCode': 12,  '2.registrationDetail.rentalParkFlag': '',  '2.registrationDetail.seminoleMiccosukeeIndianFlag': false,  '2.registrationDetail.taxCollectorRenewalEligibilityFlag': true,  '2.registrationDetail.vehicleClassCode': 1,  '2.registrationOwners.0customer.addresses.0addressType': 'R',  '2.registrationOwners.0customer.addresses.0city': 'SARASOTA',  '2.registrationOwners.0customer.addresses.0countryCode': 'US',  '2.registrationOwners.0customer.addresses.0countyCode': 12,  '2.registrationOwners.0customer.addresses.0foreignPostalCode': '',  '2.registrationOwners.0customer.addresses.0state': 'FL',  '2.registrationOwners.0customer.addresses.0streetAddress': '39875 44TH DR E',  '2.registrationOwners.0customer.addresses.0zipCode': 34243,  '2.registrationOwners.0customer.addresses.0zipPlus': 5566,  '2.registrationOwners.0customer.addresses.1addressType': 'M',  '2.registrationOwners.0customer.addresses.1city': 'PALMETTO',  '2.registrationOwners.0customer.addresses.1countryCode': 'US',  '2.registrationOwners.0customer.addresses.1countyCode': 12,  '2.registrationOwners.0customer.addresses.1foreignPostalCode': '',  '2.registrationOwners.0customer.addresses.1state': 'FL',  '2.registrationOwners.0customer.addresses.1streetAddress': '39875 44TH DR E',  '2.registrationOwners.0customer.addresses.1zipCode': 34221,  '2.registrationOwners.0customer.addresses.1zipPlus': '',  '2.registrationOwners.0customer.companyName': '',  '2.registrationOwners.0customer.customerNumber': 2398574,  '2.registrationOwners.0customer.customerStopFlag': false,  '2.registrationOwners.0customer.customerType': 'I',  '2.registrationOwners.0customer.dateOfBirth': '1958-01-11T00:00:00',  '2.registrationOwners.0customer.dlExpirationDate': '2020-01-11T00:00:00',  '2.registrationOwners.0customer.dlRenewalEligibilityFlag': true,  '2.registrationOwners.0customer.driverLicenseNumber': 'B23987433',  '2.registrationOwners.0customer.emailAddress': '',  '2.registrationOwners.0customer.feidNumber': '',  '2.registrationOwners.0customer.firstName': 'DAVID2',  '2.registrationOwners.0customer.lastName': 'HUGH2',  '2.registrationOwners.0customer.middleName': 'BRIERTON2',  '2.registrationOwners.0customer.militaryExemptionFlag': false,  '2.registrationOwners.0customer.nameSuffix': '',  '2.registrationOwners.0customer.sex': 'M',  '2.registrationOwners.0registrationOwnershipNumber': 1,  '2.shippingAddress.address.addressType': 'S',  '2.shippingAddress.address.city': 'PALMETTO',  '2.shippingAddress.address.countryCode': 'US',  '2.shippingAddress.address.countyCode': 12,  '2.shippingAddress.address.foreignPostalCode': '',  '2.shippingAddress.address.state': 'FL',  '2.shippingAddress.address.streetAddress': '293847 33TH ST W',  '2.shippingAddress.address.zipCode': 34221,  '2.shippingAddress.address.zipPlus': '',  '2.shippingAddress.shippingCompanyName': '',  '2.shippingAddress.shippingName1': 'DAVID2 HUGH2 BRIERTON2',  '2.shippingAddress.shippingName2': '',  '2.stops': '',  '2.vehicle.address': '',  '2.vehicle.bodyCode': '2D',  '2.vehicle.brakeHorsePower': '',  '2.vehicle.cubicCentimeters': '',  '2.vehicle.grossWeight': '',  '2.vehicle.insuranceAffidavitCode': '',  '2.vehicle.leaseOwnerFlag': false,  '2.vehicle.lengthFeet': '',  '2.vehicle.lengthInches': '',  '2.vehicle.majorColorCode': 'BLK',  '2.vehicle.makeCode': 'PONT',  '2.vehicle.minorColorCode': '',  '2.vehicle.netWeight': 3802,  '2.vehicle.numberOfAxles': '',  '2.vehicle.ownerUnitNumber': '',  '2.vehicle.titleNumber': 239857424,  '2.vehicle.vehicleIdentificationNumber': '6G23242312UX63297437',  '2.vehicle.vehicleNumber': '35T7843',  '2.vehicle.vehicleType': 'AU',  '2.vehicle.vesselCode': '',  '2.vehicle.vesselManufacturerDesc': '',  '2.vehicle.vesselResidentStatus': 'Y',  '2.vehicle.vesselWaterType': '',  '2.vehicle.widthFeet': '',  '2.vehicle.widthInches': '',  '2.vehicle.yearMake': 2006 ]

You can use something like below

data = require("./data.json")flattenObject = (obj) => {    let flattenKeys = {};    for (let i in obj) {        if (!obj.hasOwnProperty(i)) continue;        if ((typeof obj[i]) == 'object') {            // flattenKeys[i] = obj[i];            let flatObject = flattenObject(obj[i]);            for (let j in flatObject) {                if (!flatObject.hasOwnProperty(j)) continue;                flattenKeys[i + '.' + j] = flatObject[j];            }        } else {            flattenKeys[i] = obj[i];        }    }    return flattenKeys;}console.log(flattenObject(data))

The output on first element of your array object is as below

{ countyCode: 12,  'customer.addresses.0.addressType': 'R',  'customer.addresses.0.city': 'BRADENTON',  'customer.addresses.0.countryCode': 'US',  'customer.addresses.0.countyCode': 12,  'customer.addresses.0.state': 'FL',  'customer.addresses.0.streetAddress': '819 15th Ave Dr E',  'customer.addresses.0.zipCode': 34211,  'customer.addresses.1.addressType': 'M',  'customer.addresses.1.city': 'BRADENTON',  'customer.addresses.1.countryCode': 'US',  'customer.addresses.1.countyCode': 12,  'customer.addresses.1.state': 'FL',  'customer.addresses.1.streetAddress': 'PO BOX 124',  'customer.addresses.1.zipCode': 34201,  'customer.addresses.1.zipPlus': '124',  'customer.customerNumber': 932874,  'customer.customerStopFlag': false,  'customer.customerType': 'I',  'customer.dateOfBirth': '1936-08-05T00:00:00',  'customer.dlExpirationDate': '2022-08-05T00:00:00',  'customer.dlRenewalEligibilityFlag': true,  'customer.driverLicenseNumber': 'B360722339284',  'customer.firstName': 'David',  'customer.lastName': 'Brierton',  'customer.middleName': 'Hugh',  'customer.sex': 'M' }

Another solution:

function flatObjectToString(obj) {    var path = [],        nodes = {},        parseObj = function (obj) {            if (typeof obj == 'object') {                if (obj instanceof Array) { //array                    for (var i = 0, l = obj.length; i < l; i++) {                        path.push(i);                        parseObj(obj[i]);                        path.pop();                    }                }                else {  //object                    for (var node in obj) {                        path.push(node);                        parseObj(obj[node]);                        path.pop();                    }                }            }            else {  //value                nodes[path.join('_')] = obj;            }        };    parseObj(obj);    return nodes;}console.log(JSON.stringify(flatObjectToString(data)));

• I used a global path array to store the recursive path and prevent a lot a string operations
• I don't know about the size of the actual JSON files so I tried to get a solution which doesn't waste any garbage
• There is no need to check for "hasOwnProperty" in this (your) case