_updateCssClassName () {
    if (this.dom.field &&
      this.editor &&
      this.editor.options &&
      typeof this.editor.options.onClassName === 'function' &&
      this.dom.tree) {
      removeAllClassNames(this.dom.tree)
      const getValue = this.getValue.bind(this)
      const addClasses = this.editor.options.onClassName({
        path: this.getPath(),
        field: this.field,
        get value () {
          return getValue()
        }
      }) || ''
      addClassName(this.dom.tree, 'jsoneditor-values ' + addClasses)
    }
  }
set custom css classes on a node