let objs = [
	{'id': '1', 'firstName': 'Alfred', 'lastName': 'Schmidt'},
	{'id': '2', 'firstName': 'Howard', 'lastName': 'Snyder'},
	{'id': '3', 'firstName': 'Yoshi', 'lastName': 'Latimer'}
];

//optional
let getThis = [
	'id',
	'firstName'//,
	//'lastName'
];

//optional
let header = [
	'ID',
	'First name',
	'Last name'
];
/*

OR
let header = 1;

*/

console.log( jsonToCSV(objs) );
console.log( jsonToCSV(objs, 1) );
console.log( jsonToCSV(objs, header) );
console.log( jsonToCSV(objs, header, getThis) );