I want to load an owl.file with rdflib.js based on this tutorial but get the following error:

Error: Error: NamedNode IRI "/ontologies/opcua.owl" must be absolute. while trying to parse </ontologies/opcua.owl> as application/rdf+xml

I "installed" rdflib.js to use in the Browser (with generating a "script" file)
Excerpt of the HTML-File

<script src="rdflib.js/dist/rdflib.min.js"></script>
<script src="javascript/OWL.js"></script>

Javascript Code

const store = $rdf.graph();

var uri = ('/ontologies/opcua.owl').toString();
var mimeType = 'application/rdf+xml';

var body

try {
    $rdf.parse(body, store, uri, mimeType)
} catch (err) {
    console.log(err)
}

Thanks in advance for every hint or remarks to my code.