为了提高页面加载性能，在页面引入外部js的时候，可以使用script标签的defer和async属性来使得外部js异步加载
script标签的使用分为三种情况：
- 1.
<script src="demo.js"></script>
没有defer或async属性，浏览器会立即加载并执行相应的脚本。也就是说在渲染script标签之后的文档之前，不等待后续加载的文档元素，读到就开始加载和执行，此举会阻塞后续文档的加载；
- 2.
<script async src="demo.js"></script>
有了async属性，表示后续文档的加载和渲染与js脚本的加载和执行是并行进行的，即异步执行；
- 3.
<script defer src="demo.js"></script>
有了defer属性，加载后续文档的过程和js脚本的加载(此时仅加载不执行)是并行进行的(异步)，js脚本的执行需要等到文档所有元素解析完成之后，DOMContentLoaded事件触发执行之前。
下图可以直观的看出三者之间的区别:
其中蓝色代表js脚本网络加载时间，红色代表js脚本执行时间，绿色代表html解析。
从图中我们可以明确几点：
defer和async在网络加载过程是一致的，都是异步执行的；
两者的区别在于脚本加载完成之后何时执行，可以看出defer更符合大多数场景对应用脚本加载和执行的要求；
如果存在多个有defer属性的脚本，那么它们是按照加载顺序执行脚本的；而对于async，它的加载和执行是紧紧挨着的，无论声明顺序如何，只要加载完成就立刻执行，它对于应用脚本用处不大，因为它完全不考虑依赖。
小结：
defer 和 async 的共同点是都是可以并行加载JS文件，不会阻塞页面的加载，不同点是 defer的加载完成之后，JS会等待整个页面全部加载完成了再执行，而async是加载完成之后，会马上执行JS，所以假如对JS的执行有严格顺序的话，那么建议用 defer加载。
参考： use_async_or_defer
In general, you should always use async or defer for third party scripts (unless the script does something necessary for the critical rendering path):
Use async if it's important to have the script run earlier in the loading process. This might include some analytics scripts, for example.
Use defer for less critical resources. A video player that's below-the-fold, for example.
Note: If performance is your primary concern, you could wait until your page has reached a key user moment (such as after the critical content has loaded) before adding async scripts. You should also take care not to async load libraries like jQuery just because they are coming from a third-party CDN.
Note: In Blink-based browsers,
asyncanddefercurrently lower the priority of the network request for resources so it can cause it to load significantly later than it would as a blocking script. This is useful to know in particular for analytics scripts.
Should you ever load third-party scripts without async or defer? You could make a case for this if the script is a crucial part of your site functionality. For example, if you're loading your main UI library or framework from a CDN, it will be badged as "third-party script" in DevTools, but should be treated as an essential part of your site, not an add-on.
Note that not all scripts work if loaded asynchronously. Check the docs for any third-party scripts you're using. If you're using a script that can't be loaded asynchronously, you might want to consider an alternative, or eliminating the script if possible. Some third parties may highly recommend to load their scripts sync (to get ahead of other scripts), even if they would work fine async so do due diligence when evaluating strategies for loading third-party scripts.
