xxxxxxxxxx
<script async src="https://www.googletagmanager.com/gtag/js?id=G-VB19CSV8XL"></script>
xxxxxxxxxx
<html>
<head>
</head>
<body>
<script type="text/javascript" src=es4.js></script>
</body>
</html>
xxxxxxxxxx
<script type="text/javascript" src="path-to-javascript-file.js"></script>
xxxxxxxxxx
//put it in last of body so javascript load after html element and javascript
//can change html element
<body>
<script src="index.js"></script>
</body>
xxxxxxxxxx
import('your-file-name.js')
//this only works for javascript files
//and with bugs or half bugs it won't work
xxxxxxxxxx
check this link:
https://stackoverflow.com/questions/950087/how-do-i-include-a-javascript-file-in-another-javascript-file
it wil tell you it in all js variants (including jQuery and node.js)