// Initialize the Facebook SDK.
FB.init({
  appId: '<your-app-id>',
  oauth: true
  /* additional properties */
});

// Open login popup.
FB.login(function(response) {
  if(response.authResponse) {
    // Perform some logic to save the user and access token.
  }
  else {
    // User cancelled login or did not fully authorize.
  }
});