I think you are using @jeffz solid-file-client lib…
Be careful, this lib is for dealing with resource (file or folder). And you risk to delete your card (profile). It’s quite hard to manage content of the file with sfc, just replace all content.
To deal with resource in your profile/card file I suggest you to take a look at @RubenVerborgh query-ldflex or @Vincent tripledoc .
Solid-File-Client deals with Solid resources as complete documents, it does not operate on the contents of documents, e.g. RDF triples. It is useful to read, upload, download, copy, move, or replace documents and folders, not to edit their details. The code sample you show would have the effect of erasing the entire profile document pointed to by webId. That would cause serious problems, so it would be better to use a finer-grained tool like rdflib, ldflex, or tripledoc.