为什么我不能使用gapi.client.gmail.users.labels.list？

Question

我的变量＆＃39; gapi.client.gmail.users.labels.list＆＃39;并不等于Null，但我无法解决这个问题。错误文本是

  

未捕获的TypeError：无法读取属性＆＃39;用户＆＃39;未定义的

我该如何解决这个错误???

有我的代码：

var API_KEY = 'xxxxxxxxxxxxxxxxxxxxxxx'; 
var OAUTH2_CLIENT_ID = 'xxxxxxxxxxxxxxxxxxxxx.googleusercontent.com';
var OAUTH2_SCOPES = [
  'https://www.googleapis.com/auth/youtube',
  'https://www.googleapis.com/auth/gmail.readonly'
];
var DISCOVERY_DOCS = ["https://www.googleapis.com/discovery/v1/apis/gmail/v1/rest"];

  var GoogleAuth;
  var SCOPE = 'https://www.googleapis.com/auth/youtube.force-ssl';
  function handleClientLoad() {
    // Load the API's client and auth2 modules.
    // Call the initClient function after the modules load.
    gapi.load('client:auth2', initClient);
  }

  function initClient() {
    // Retrieve the discovery document for version 3 of YouTube Data API.
    // In practice, your app can retrieve one or more discovery documents.
    var discoveryUrl = 'https://www.googleapis.com/discovery/v1/apis/youtube/v3/rest';

    // Initialize the gapi.client object, which app uses to make API requests.
    // Get API key and client ID from API Console.
    // 'scope' field specifies space-delimited list of access scopes.
    gapi.client.init({
        'apiKey': API_KEY,
        'discoveryDocs': [discoveryUrl],
        'clientId': OAUTH2_CLIENT_ID,
        'scope': SCOPE
    }).then(function () {
      GoogleAuth = gapi.auth2.getAuthInstance();

      // Listen for sign-in state changes.
      GoogleAuth.isSignedIn.listen(updateSigninStatus);

      // Handle initial sign-in state. (Determine if user is already signed in.)
      var user = GoogleAuth.currentUser.get();
      setSigninStatus();

      // Call handleAuthClick function when user clicks on
      //      "Sign In/Authorize" button.
      $('#sign-in-or-out-button').click(function() {
        handleAuthClick();
      }); 
      $('#revoke-access-button').click(function() {
        revokeAccess();      
      }); 
    });
  }

  function handleAuthClick() {
    if (GoogleAuth.isSignedIn.get()) {
      // User is authorized and has clicked 'Sign out' button.
      GoogleAuth.signOut();
    } else {
      // User is not signed in. Start Google auth flow.
      GoogleAuth.signIn();
    }
  }

  function revokeAccess() {
    GoogleAuth.disconnect();
  }

  function setSigninStatus(isSignedIn) {
    var user = GoogleAuth.currentUser.get();
    var isAuthorized = user.hasGrantedScopes(SCOPE);
    if (isAuthorized) {
      $('#sign-in-or-out-button').html('Sign out');
      $('#revoke-access-button').css('display', 'inline-block');
      $('#auth-status').html('You are currently signed in and have granted ' +
          'access to this app.');
    } else {
      $('#sign-in-or-out-button').html('Sign In/Authorize');
      $('#revoke-access-button').css('display', 'none');
      $('#auth-status').html('You have not authorized this app or you are ' +
          'signed out.');
    }
    gapi.client.gmail.users;
  }

  function updateSigninStatus(isSignedIn) {
    setSigninStatus();
  }
  function gmailInit()
  {
      gapi.client.init({
          discoveryDocs: ["https://www.googleapis.com/discovery/v1/apis/gmail/v1/rest"],
          clientId: OAUTH2_CLIENT_ID,
          scope: OAUTH2_SCOPES
        });
        gapi.client.gmail.users.labels.list({
          'userId': 'me'
        }).then(function(response) {
          var labels = response.result.labels;
          appendPre('Labels:');

          if (labels && labels.length > 0) {
            for (i = 0; i < labels.length; i++) {
              var label = labels[i];
              appendPre(label.name)
            }
          } else {
            appendPre('No Labels found.');
          }
        });
  }
$(window).load(function() {
  $('#gmail-init-button').on('click',function() {
    gmailInit();      
  });
});

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