demo.html

<script src="https://apis.google.com/js/api.js"></script>
<script>
  /**
   * Sample JavaScript code for calendar.calendarList.list
   * See instructions for running APIs Explorer code samples locally:
   * https://developers.google.com/explorer-help/code-samples#javascript
   */

  function authenticate() {
    return gapi.auth2.getAuthInstance()
        .signIn({scope: "https://www.googleapis.com/auth/calendar https://www.googleapis.com/auth/calendar.readonly", plugin_name: 'dinnerwithfriends'})
        .then(function() { console.log("Sign-in successful"); },
              function(err) { console.error("Error signing in", err); });
  }
  function loadClient() {
    gapi.client.setApiKey("AIzaSyA7G2ANAJI6rm_DpTW84lsKUJT-c8bmirI");
    return gapi.client.load("https://content.googleapis.com/discovery/v1/apis/calendar/v3/rest")
        .then(function() { console.log("GAPI client loaded for API");
        execute()
      },
              function(err) { console.error("Error loading GAPI client for API", err); });
  }
  // Make sure the client is loaded and sign-in is complete before calling this method.
  function execute() {
    return gapi.client.calendar.calendarList.list({})
        .then(function(response) {
                // Handle the results here (response.result has the parsed body).
                console.log("Response", response);
              },
              function(err) { console.error("Execute error", err); });
  }
  gapi.load("client:auth2", function() {
    gapi.auth2.init({client_id: "102076896830-4il8ncmrd6qfoippk2ut4uujb8cci54v.apps.googleusercontent.com"});
  });
</script>
<button onclick="authenticate().then(loadClient)">authorize and load</button>
<button onclick="execute()">execute</button>