Why does .join Method not work on my array?

I want to use JS to create a XML file, for thus I want to print my array that contains the snippets of my XML file without commas but .join() doesnt work

// creating of the XML snippets “<test” and “>”

let testArray = [];
var XML = new XMLWriter();
XML.BeginNode("test");
XML.EndNodeSingle();

// pushing into array and using .join()

testArray.push(XML.XML);

testArray = testArray.join(' ');

// trying to print array into a textarea

document.getElementById('xml').innerHTML = testArray;

// returns <test,>” but <test> is needed