The src path is relative to the html file. If both are in the same directory, then try:
<script type="text/javascript" src="https://stackoverflow.com/questions/31767363/Application.js"></script>
if in a subdirectory
<script type="text/javascript" src="https://stackoverflow.com/questions/31767363/dirName/Application.js"></script>
if in a parent directory
<script type="text/javascript" src="https://stackoverflow.com/questions/31767363/Application.js"></script>
However, make sure that the JS file is somewhere in the hierarchy of the root directory of your website, that is the root folder that your web server uses.
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solved Can’t attach local Javascript file to HTML