from MDN:
https://developer.mozilla.org/en-US/docs/Web/API/HTMLHyperlinkElementUtils/protocol

in html, protocol is usually equal to https:, http:, file: and so on.

but I found in https://github.com/sockjs/sockjs-client/blob/master/lib/main.js#L78

if (loc.protocol === 'https' && !secure) {
    throw new Error('SecurityError: An insecure SockJS connection may not be initiated from a page loaded over HTTPS');
  }

where the final ":" is missed