Google Calendar API CalendarList.list() returns a weird object - JS

I want to get the list of the calendars of a user with JS by using CalendarList.list() as stated in the documentation. But for me I only get a weird object as response, that always looks like this:

{
    "Yd": 1,
    "mb": {
        "Vf": null,
        "yh": {
            "path": "/calendar/v3/users/me/calendarList",
            "method": "GET",
            "params": {},
            "headers": {},
            "root": "https://www.googleapis.com",
            "apiId": "calendar:v3"
        },
        "Cl": "auto",
        "G_": false,
        "VN": false,
        "aQ": false
    }
}

I'm using the standard code from the quickstart which works fine for me, only this one request returns a weird result.

It looks like this in my code (just the part of it that should be interesting):

function initClient() {
      gapi.client.init({
        apiKey: API_KEY,
        clientId: CLIENT_ID,
        discoveryDocs: DISCOVERY_DOCS,
        scope: SCOPES
      }).then(function() {
        // Listen for sign-in state changes.
        gapi.auth2.getAuthInstance().isSignedIn.listen(updateSigninStatus);

        // Handle the initial sign-in state.
        updateSigninStatus(gapi.auth2.getAuthInstance().isSignedIn.get());
        authorizeButton.onclick = handleAuthClick;
        signoutButton.onclick = handleSignoutClick;
      }, function(error) {
        changeSpanText(JSON.stringify(error, null, 2));
      });
    }


function updateSigninStatus(isSignedIn) {
      if (isSignedIn) {
        authorizeButton.style.display = 'none';
        signoutButton.style.display = 'flex';
        changeSpanText("🎉 You have been logged in succesfully");
        console.log(gapi.client.calendar.calendarList.list());
      } else {
        authorizeButton.style.display = 'flex';
        signoutButton.style.display = 'none';
        changeSpanText("");
      }
    }