why shouldn't we put any javascript code between the <script> tag if we only want to include external js file?
<p>i don't really understand why the author in the "Javascript &amp; Jquery: the missing manual , second edition" advise the sentence below:</p>

<blockquote>
  <p>When adding the src attribute to link to an external JavaScript file, don’t add any JavaScript code
  between the opening and closing  tags. If you want to link to an external JavaScript file and add
  custom JavaScript code to a page, use a second set of  tags. For example:</p>
</blockquote>

<p>what is the advantage of using the second script tag instead of only using one?</p>
