snabbdom/modules/class.js

function updateClass(oldVnode, vnode) {
  var cur, name, elm = vnode.elm,
      oldClass = oldVnode.data.class || {},
      klass = vnode.data.class || {};
  for (name in klass) {
    cur = klass[name];
    if (cur !== oldClass[name]) {
      elm.classList[cur ? 'add' : 'remove'](name);
    }
  }
}

module.exports = {create: updateClass, update: updateClass};