Edit in JSFiddle

document.querySelector('.target').addEventListener('click', function(event) {
  event.preventDefault();
  var target = document.querySelector('.target');
  if (target.classList.contains('active')) {
    target.classList.remove('active');
  } else {
    target.classList.add('active');
  }
});

<button class="target">click to add/remove class</button>

.active {
  font-weight: bold;
  color: red;
}