run script before page load
//Use the defer attribute on a classic script tag:
<script defer src="./my-code.js"></script>
//Use JavaScript modules. A type="module" script is deferred until the
//HTML has been fully parsed and the initial DOM created.
<script type="module" src="./my-code.js"></script>
<!-- Or -->
<script type="module">
// Your code here
</script>
//Add the script to the bottom of the <body> right before the </body>.
<!doctype html>
<html>
<!-- ... -->
<body>
<!-- The document's HTML goes here -->
<script type="module" src="./my-code.js"></script><!-- Or inline script -->
</body>
</html>