To assuage your immediate fears, in an HTTPS (that is, SSL) exchange, everything in the exchange is encrypted, although you do not see the encryption/decryption being performed.   Thus, the token is not visible to anyone else . . .

. . . I - F . . . you are absolutely certain that the exchange is taking place through SSL as it should be, and as certainly appears to be the case here.   However, it still pays to make sure, for example in the server-side code (and, really, in client-side as well) that any exchanges which you expect to be encrypted, actually are.   Some systems have been exploited, in part, by sending login-request API-calls to non-encrypted versions of URLs and finding that a valid access token was returned along a wide-open channel.   Even your client-side JS code should not accept a JSON-response that came from an open channel.   (“Trust, but Verify.™”)