code: function updateClass (oldVnode, vnode) {
    const el = vnode.elm;
    const data = vnode.data;
    const oldData = oldVnode.data;
    if (
      isUndef(data.staticClass) &&
      isUndef(data.class) && (
        isUndef(oldData) || (
          isUndef(oldData.staticClass) &&
          isUndef(oldData.class)
        )
      )
    ) {
      return
    }
  
    let cls = genClassForVnode(vnode);
  
    // handle transition classes
    const transitionClass = el._transitionClasses;
    if (isDef(transitionClass)) {
      cls = concat(cls, stringifyClass(transitionClass));
    }
  
    // set the class
    if (cls !== el._prevClass) {
      el.setAttribute('class', cls);
      el._prevClass = cls;
    }
  }
docs: /*