title: How to dynamic load html file and execute the exernal js in it
body: <p>I have an html file:index.html which contains some basic js libs.
  And I also have some modules：</p>
  
  <p>a.html:</p>
  
  <pre><code>&lt;div id='a'&gt;
    &lt;script src="a1.js"&gt;&lt;/script&gt;
    &lt;script src="a2.js"&gt;&lt;/script&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>a1.js:</p>
  
  <pre><code>console.log('a1')
  </code></pre>
  
  <p>a2.js:</p>
  
  <pre><code>console.log('a2')
  </code></pre>
  
  <p>How can I dynamically load a.html into index.html and get the event that a1.js and a2.js are completed executed? </p>