Answers for "how to append class in javascript"

js add class

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

js append class

function myFunction() {
  var element = document.getElementById("myDIV");
  element.classList.add("mystyle");
}

js add class

var someElement= document.getElementById("myElement");
    someElement.className += " newclass";//add "newclass" to element (space in front is important)