Add a class to a DIV with javascript

Question

Example HTML:

How can I add the class class_two without replacing

Answer 1

You can make life easier with a couple of helper functions:

function addClass(el, className) {
  var classes = el.className.match(/\S+/g) || [];  

  if (!hasClass(el, className)) {
    classes.push(className);
  }
  el.className = classes.join(' ');
}

function hasClass(el, className) {
  var re = new RegExp('(^|\\s+)' + className + '(\\s+|$)');
  return re.test(el.className);
}

addClass(document.getElementById('foo'), 'bar')