JavaScript Module & CORS error

Because of the way your browser implements JavaScript modules this demo page needs to be loaded from a web server over HTTP to work. You could upload the files somewhere, or perhaps use a local web server like python3 -m http.server or python -m SimpleHTTPServer or ruby -run -e httpd . or http-server from npm, or osws -m -d . — or any of the others from this big list of HTTP static server one-liners.

You could also load this page in Firefox, which permits local files to see their neighbours, or start Chrome with the --allow-file-access-from-files command-line option.

In practical use, if local-file access is important, you can remove the last two lines from jq.js (the ones starting export) and then use

<script src="jq.js"></script>
<script>
let func = jq.compile(".x[].y")
for (let x of func(obj)) {
...
</script>

as usual.