why my external javascript not working code example

Example: external javascript file not working

Most likely, the problem is that you are including your js file in a head tag or somewhere above your main content. ... You should be able to add the js file in a script tag. The page loads first then JavaScript. You need conditionals to load an external script if the JavaScript file is in the head tag.
<h1> asdlfasdjfd </h1>
<script type...... src='.....js'></script>