This post shows you how to fix 'popper.js in bootstrap 4 gives SyntaxError Unexpected token export' in javascript.
For example:
<script src="libs/popper.js/dist/popper.js"></script>
Uncaught syntaxError: Unexpected token export
To solve the problem, you can add bootstrap.bundle.min.js and remove popper.js in your html file.
Because bootstrap.bundle.min.js included popper.js