用一个原生的JS对象去描述一个节点，比创建一个DOM的代价要小很多
    在Vue.js中Virtual DOM是用VNode这一个Class描述的
    定义在src/core/vdom/vnode.js 中

    1. export default class VNode {
    2.   tag: string | void; // 标签名
    3.   data: VNodeData | void; // 数据
    4.   children: ?Array<VNode>; // 子节点
    5.   text: string | void; // 文本
    6.   elm: Node | void;
    7.   ns: string | void;
    8.   context: Component | void; // rendered in this component's scope
    9.   key: string | number | void; // 键值
    10.   componentOptions: VNodeComponentOptions | void;
    11.   componentInstance: Component | void; // component instance
    12.   parent: VNode | void; // component placeholder node
    14.   // strictly internal
    15.   raw: boolean; // contains raw HTML? (server only)
    16.   isStatic: boolean; // hoisted static node
    17.   isRootInsert: boolean; // necessary for enter transition check
    18.   isComment: boolean; // empty comment placeholder?
    19.   isCloned: boolean; // is a cloned node?
    20.   isOnce: boolean; // is a v-once node?
    21.   asyncFactory: Function | void; // async component factory function
    22.   asyncMeta: Object | void;
    23.   isAsyncPlaceholder: boolean;
    24.   ssrContext: Object | void;
    25.   fnContext: Component | void; // real context vm for functional nodes
    26.   fnOptions: ?ComponentOptions; // for SSR caching
    27.   devtoolsMeta: ?Object; // used to store functional render context for devtools
    28.   fnScopeId: ?string; // functional scope id support
    30.   constructor (
    31.     tag?: string,
    32.     data?: VNodeData,
    33.     children?: ?Array<VNode>,
    34.     text?: string,
    35.     elm?: Node,
    36.     context?: Component,
    37.     componentOptions?: VNodeComponentOptions,
    38.     asyncFactory?: Function
    39.   ) {
    40.     this.tag = tag
    41.     this.data = data
    42.     this.children = children
    43.     this.text = text
    44.     this.elm = elm
    45.     this.ns = undefined
    46.     this.context = context
    47.     this.fnContext = undefined
    48.     this.fnOptions = undefined
    49.     this.fnScopeId = undefined
    50.     this.key = data && data.key
    51.     this.componentOptions = componentOptions
    52.     this.componentInstance = undefined
    53.     this.parent = undefined
    54.     this.raw = false
    55.     this.isStatic = false
    56.     this.isRootInsert = true
    57.     this.isComment = false
    58.     this.isCloned = false
    59.     this.isOnce = false
    60.     this.asyncFactory = asyncFactory
    61.     this.asyncMeta = undefined
    62.     this.isAsyncPlaceholder = false
    63.   }
    65.   // DEPRECATED: alias for componentInstance for backwards compat.
    66.   /* istanbul ignore next */
    67.   get child (): Component | void {
    68.     return this.componentInstance
    69.   }

    Vue.js中Virtual DOM是借鉴了一个开源库https://github.com/snabbdom/snabbdom的实现，然后加入了一些Vue.js特色的东西