Answers for "What property can you use to change an element's class name with JavaScript?"

js set class

var element = document.getElementById('element');
element.classList.add('class-1');
element.classList.add('class-2', 'class-3');
element.classList.remove('class-3');

Javascript change element class

document.getElementById("myElementID").classList.add('myClassName');
document.getElementById("myElementID").classList.remove('myClassName');