一段代码

    <div id="app">{{ msg }}</div>
    
    <script type="text/javascript">
        const vm = new Vue({
            el: '#app',
            data: {
                msg: "a",
            }
        });
        
        vm.msg = "b"
        Promise.resolve().then(() => {
            console.log('xxx')
        })
        vm.$nextTick(()=>{
            console.log(vm.$el.innerHTML)
        })
    </script>

控制台依次打印： b xxx

• 为什么会先打印b再打印xxx呢？nextTick不也是优先用Promise实现的嘛？既然都是进微队列那么这个现象怎么解释？

vm.msg = "b"执行; 数据变化，页面要重新渲染，但vue为了提高渲染效率，使用异步更新dom的策略，而异步更新dom实现方式主要就是调用nextTick函数。nextTick内部通过Promise.resolve().then(flushCallbacks)的方式，把flushCallbacks函数推入微队列（flushCallbacks函数的作用就是遍历执行callbacks数组所有回调函数）。

也就是说，vm.msg = "b"执行完，微队列已经有flushCallbacks函数，当你手动调用nextTick时，可以简单认为只是把回调函数push进callbacks数组里。从微队列顺序来说，你写的$nextTick回调就在Promise回调之前执行了。

简单来说，是因为数据变动时，vue内部调用了nextTick函数（表面nextTick要晚些执行，实际人家在vm.msg="b"的时候就执行了）我们把vm.msg = "b"往后移下：

    Promise.resolve().then(() => {
        console.log('xxx')
    })
    vm.msg = "b"
    vm.$nextTick(()=>{
        console.log(vm.$el.innerHTML)
    })

控制台依次打印： xxx b

当然，如果没有vm.msg = "b"这句，nextTick()就不会提前被调用，也是先输出xxx，再输出b。

-[扩展] 如果vm.$nextTick()不传参数，就可以当Promise使用

    vm.$nextTick().then(() => {
      console.log('aaaa'); 
    })

下面附上源码，供大家学习参考：

• nextTick源码：

（位置：src/core/util/next-tick.js）

    /* @flow */
    /* globals MutationObserver */
    
    import { noop } from 'shared/util'
    import { handleError } from './error'
    import { isIE, isIOS, isNative } from './env'
    
    export let isUsingMicroTask = false
    
    const callbacks = []
    let pending = false
    
    function flushCallbacks () {
      pending = false
      const copies = callbacks.slice(0)
      callbacks.length = 0
      for (let i = 0; i < copies.length; i++) {
        copies[i]()
      }
    }
    
    let timerFunc
    

    if (typeof Promise !== 'undefined' && isNative(Promise)) {
      const p = Promise.resolve()
      timerFunc = () => {
        p.then(flushCallbacks)
        // In problematic UIWebViews, Promise.then doesn't completely break, but
        // it can get stuck in a weird state where callbacks are pushed into the
        // microtask queue but the queue isn't being flushed, until the browser
        // needs to do some other work, e.g. handle a timer. Therefore we can
        // "force" the microtask queue to be flushed by adding an empty timer.
        if (isIOS) setTimeout(noop)
      }
      isUsingMicroTask = true
    } else if (!isIE && typeof MutationObserver !== 'undefined' && (
      isNative(MutationObserver) ||
      // PhantomJS and iOS 7.x
      MutationObserver.toString() === '[object MutationObserverConstructor]'
    )) {
      // Use MutationObserver where native Promise is not available,
      // e.g. PhantomJS, iOS7, Android 4.4
      // (#6466 MutationObserver is unreliable in IE11)
      let counter = 1
      const observer = new MutationObserver(flushCallbacks)
      const textNode = document.createTextNode(String(counter))
      observer.observe(textNode, {
        characterData: true
      })
      timerFunc = () => {
        counter = (counter + 1) % 2
        textNode.data = String(counter)
      }
      isUsingMicroTask = true
    } else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
      // Fallback to setImmediate.
      // Technically it leverages the (macro) task queue,
      // but it is still a better choice than setTimeout.
      timerFunc = () => {
        setImmediate(flushCallbacks)
      }
    } else {
      // Fallback to setTimeout.
      timerFunc = () => {
        setTimeout(flushCallbacks, 0)
      }
    }
    
    export function nextTick (cb?: Function, ctx?: Object) {
      let _resolve
      callbacks.push(() => {
        if (cb) {
          try {
            cb.call(ctx)
          } catch (e) {
            handleError(e, ctx, 'nextTick')
          }
        } else if (_resolve) {
          _resolve(ctx)
        }
      })
      if (!pending) {
        pending = true
        timerFunc()
      }
      // $flow-disable-line
      if (!cb && typeof Promise !== 'undefined') {
        return new Promise(resolve => {
          _resolve = resolve
        })
      }
    }