OAuth 2.0 for Client-side Web Applications

var GoogleAuth; // Google Auth object.
function initClient() {
  gapi.client.init({
      'apiKey': 'YOUR_API_KEY',
      'clientId': 'YOUR_CLIENT_ID',
      'scope': 'https://www.googleapis.com/auth/drive.metadata.readonly',
      'discoveryDocs': ['https://www.googleapis.com/discovery/v1/apis/drive/v3/rest']
  }).then(function () {
      GoogleAuth = gapi.auth2.getAuthInstance();

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

GoogleAuth.signIn();

var isAuthorized;
var currentApiRequest;

/**
 * Store the request details. Then check to determine whether the user
 * has authorized the application.
 *   - If the user has granted access, make the API request.
 *   - If the user has not granted access, initiate the sign-in flow.
 */
function sendAuthorizedApiRequest(requestDetails) {
  currentApiRequest = requestDetails;
  if (isAuthorized) {
    // Make API request
    // gapi.client.request(requestDetails)

    // Reset currentApiRequest variable.
    currentApiRequest = {};
  } else {
    GoogleAuth.signIn();
  }
}

/**
 * Listener called when user completes auth flow. If the currentApiRequest
 * variable is set, then the user was prompted to authorize the application
 * before the request executed. In that case, proceed with that API request.
 */
function updateSigninStatus(isSignedIn) {
  if (isSignedIn) {
    isAuthorized = true;
    if (currentApiRequest) {
      sendAuthorizedApiRequest(currentApiRequest);
    }
  } else {
    isAuthorized = false;
  }
}

// Example 1: Use method-specific function
var request = gapi.client.drive.about.get({'fields': 'user'});

// Execute the API request.
request.execute(function(response) {
  console.log(response);
});


// Example 2: Use gapi.client.request(args) function
var request = gapi.client.request({
  'method': 'GET',
  'path': '/drive/v3/about',
  'params': {'fields': 'user'}
});
// Execute the API request.
request.execute(function(response) {
  console.log(response);
});

<script>
  var GoogleAuth;
  var SCOPE = 'https://www.googleapis.com/auth/drive.metadata.readonly';
  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 Google Drive API.
    // In practice, your app can retrieve one or more discovery documents.
    var discoveryUrl = 'https://www.googleapis.com/discovery/v1/apis/drive/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': 'YOUR_API_KEY',
        'discoveryDocs': [discoveryUrl],
        'clientId': 'YOUR_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.');
    }
  }

  function updateSigninStatus(isSignedIn) {
    setSigninStatus();
  }
</script>

<button id="sign-in-or-out-button"
        style="margin-left: 25px">Sign In/Authorize</button>
<button id="revoke-access-button"
        style="display: none; margin-left: 25px">Revoke access</button>

<div id="auth-status" style="display: inline; padding-left: 25px"></div><hr>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
<script async defer src="https://apis.google.com/js/api.js" 
        onload="this.onload=function(){};handleClientLoad()" 
        onreadystatechange="if (this.readyState === 'complete') this.onload()">
</script>

// Space-separated list of additional scope(s) you are requesting access to.
// This code adds read-only access to the user's calendars via the Calendar API.
var NEW_SCOPES = 'https://www.googleapis.com/auth/calendar.readonly';

// Retrieve the GoogleUser object for the current user.
var GoogleUser = GoogleAuth.currentUser.get();
GoogleUser.grant({'scope': NEW_SCOPES});

GoogleAuth 。断开（）;