example_function_jsonToCSV.js

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) );

function_jsonToCSV.js

function jsonToCSV(objs, header = [''], getThis){
	if(objs.constructor == {}.constructor){
		objs = [objs];
	};

	if(typeof getThis == 'string'){
		getThis = [getThis];
	}else if(typeof getThis == 'undefined'){
		getThis = objs[0];
		getThis = Object.keys(getThis);
	};

	if(typeof header == 'string'){
		header = [header];
	}else if(header === 1){
		header = getThis;
	};

	let tab = '	';
	let spreadsheet = header.join(tab) + '\n';
	if(spreadsheet == '\n'){
		spreadsheet = '';
	};
	for(let i = 0; i < objs.length; i++){
		let thisObj = objs[i];
		for(let j = 0; j < getThis.length; j++){
			spreadsheet += thisObj[ getThis[j] ].toString().replace(/\t/g, '');
			if(j < getThis.length - 1){
				spreadsheet += tab;
			};
		};
		if(i < objs.length - 1){
			spreadsheet += '\n';
		};
	};
	return spreadsheet;
};