We're implementing a usernameless flow in our app. Front is made in React (using webauthn-json for convenience), backend is ASP.NET Core (using fido2-net-lib).
What we want is to have a button in the login screen, which pressed, if the user has enrolled credentials, start the assertion process. If not, simply notify there're no credentials avaliable.
Is there any way to check for enrolled credentials via Javascript?
There is not.
You can integrate with autofill, which will show any local credentials for the user, but the pattern of opening a dialog only if there are local credentials only exists on mobile, not the web, currently.