本頁面由 Cloud Translation API 翻譯而成。

建構範例應用程式

此頁面將逐步引導您建構使用多個 API 的應用程式，以檢視使用者 YouTube 影片的檢視統計資料。應用程式會執行下列工作：

其使用 YouTube Data API 擷取目前已驗證的使用者上傳影片清單，然後顯示影片標題清單。
當使用者點選特定影片時，應用程式會呼叫 YouTube Analytics API，以擷取該影片的數據分析資料。
應用程式使用 Google 視覺化 API 來分析分析資料。

下列步驟說明建構應用程式的流程。在步驟 1 中，您會建立應用程式的 HTML 和 CSS 檔案。步驟 2 至 5 說明應用程式使用的 JavaScript 的不同部分。完整程式碼範例也包含在文件結尾。

步驟 1：建立 HTML 網頁與 CSS 檔案
步驟 2：啟用 OAuth 2.0 驗證
步驟 3：擷取目前登入使用者的資料
步驟 4：要求影片的 Analytics (分析) 資料
步驟 5：在圖表中顯示 Analytics (分析) 資料

重要事項：您必須向 Google 註冊應用程式，才能取得應用程式的 OAuth 2.0 用戶端 ID。

步驟 1：建立 HTML 網頁與 CSS 檔案

在這個步驟中，您必須建立 HTML 網頁，載入應用程式要使用的 JavaScript 程式庫。以下的 HTML 程式碼為網頁的程式碼：


<!doctype html>
<html>
<head>
  <title>Google I/O YouTube Codelab</title>
  <link type="text/css" rel="stylesheet" href="index.css">
  <script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
  <script type="text/javascript" src="//www.google.com/jsapi"></script>
  <script type="text/javascript" src="index.js"></script>
  <script type="text/javascript" src="https://apis.google.com/js/client.js?onload=onJSClientLoad"></script>
</head>
<body>
  <div id="login-container" class="pre-auth">This application requires access to your YouTube account.
    Please <a href="#" id="login-link">authorize</a> to continue.
  </div>
  <div class="post-auth">
    <div id="message"></div>
    <div id="chart"></div>
    <div>Choose a Video:</div>
    <ul id="video-list"></ul>
  </div>
</body>
</html>

如範例網頁的 <head> 標記所示，應用程式會使用下列程式庫：

jQuery 提供輔助方法，可簡化 HTML 文件追蹤、事件處理、動畫和 Ajax 互動。
Google API 載入器 (www.google.com/jsapi) 可讓您輕鬆匯入一或多個 Google API。此範例應用程式使用 API 載入器載入 Google 視覺化 API，以此用來擷取所擷取的 Analytics (分析) 資料。
index.js 程式庫包含範例應用程式專用的函式。本教學課程會逐步引導您建立這些函式。
JavaScript 專用 Google API 用戶端程式庫可協助您實作 OAuth 2.0 驗證，並呼叫 YouTube Analytics API。

範例應用程式也包含 index.css 檔案。您可在 CSS 檔案所在的目錄中儲存 CSS 檔案範例：


body {
  font-family: Helvetica, sans-serif;
}

.pre-auth {
  display: none;
}

.post-auth {
  display: none;
}

#chart {
  width: 500px;
  height: 300px;
  margin-bottom: 1em;
}

#video-list {
  padding-left: 1em;
  list-style-type: none;
}
#video-list > li {
  cursor: pointer;
}
#video-list > li:hover {
  color: blue;
}

步驟 2：啟用 OAuth 2.0 驗證

在這個步驟中，您將開始建立 HTML 網頁所呼叫的 index.js 檔案。因此，請在您的 HTML 網頁所在的目錄中建立名為 index.js 的檔案，然後將下列程式碼插入該檔案中。將 YOUR_CLIENT_ID 字串換成您註冊應用程式的用戶端 ID。

(function() {

  // Retrieve your client ID from the Google API Console at
  // https://console.cloud.google.com/.
  var OAUTH2_CLIENT_ID = 'YOUR_CLIENT_ID';
  var OAUTH2_SCOPES = [
    'https://www.googleapis.com/auth/yt-analytics.readonly',
    'https://www.googleapis.com/auth/youtube.readonly'
  ];

  // Upon loading, the Google APIs JS client automatically invokes this callback.
  // See https://developers.google.com/api-client-library/javascript/features/authentication 
  window.onJSClientLoad = function() {
    gapi.auth.init(function() {
      window.setTimeout(checkAuth, 1);
    });
  };

  // Attempt the immediate OAuth 2.0 client flow as soon as the page loads.
  // If the currently logged-in Google Account has previously authorized
  // the client specified as the OAUTH2_CLIENT_ID, then the authorization
  // succeeds with no user intervention. Otherwise, it fails and the
  // user interface that prompts for authorization needs to display.
  function checkAuth() {
    gapi.auth.authorize({
      client_id: OAUTH2_CLIENT_ID,
      scope: OAUTH2_SCOPES,
      immediate: true
    }, handleAuthResult);
  }

  // Handle the result of a gapi.auth.authorize() call.
  function handleAuthResult(authResult) {
    if (authResult) {
      // Authorization was successful. Hide authorization prompts and show
      // content that should be visible after authorization succeeds.
      $('.pre-auth').hide();
      $('.post-auth').show();

      loadAPIClientInterfaces();
    } else {
      // Authorization was unsuccessful. Show content related to prompting for
      // authorization and hide content that should be visible if authorization
      // succeeds.
      $('.post-auth').hide();
      $('.pre-auth').show();

      // Make the #login-link clickable. Attempt a non-immediate OAuth 2.0
      // client flow. The current function is called when that flow completes.
      $('#login-link').click(function() {
        gapi.auth.authorize({
          client_id: OAUTH2_CLIENT_ID,
          scope: OAUTH2_SCOPES,
          immediate: false
        }, handleAuthResult);
      });
    }
  }

  // This helper method displays a message on the page.
  function displayMessage(message) {
    $('#message').text(message).show();
  }

  // This helper method hides a previously displayed message on the page.
  function hideMessage() {
    $('#message').hide();
  }
  /* In later steps, add additional functions above this line. */
})();

步驟 3：擷取目前登入使用者的資料

在這個步驟中，您要在 index.js 檔案中新增函式，以擷取目前登入使用者上傳影片，並使用 YouTube Data API (v2.0) 上傳。這個動態饋給會指定使用者的 YouTube 頻道 ID，呼叫 YouTube Analytics API 時需要用到。此外，範例應用程式也會列出使用者上傳的影片，讓使用者能夠擷取任何個別影片的 Analytics (分析) 資料。

對 index.js 檔案進行下列變更：

新增函式，用來載入 YouTube 數據分析和 Data API 的使用者介面。這是使用 Google API JavaScript 用戶端的先決條件。

載入這兩種 API 用戶端介面後，函式會呼叫 getUserChannel 函式。


  // Load the client interfaces for the YouTube Analytics and Data APIs, which
  // are required to use the Google APIs JS client. More info is available at
  // https://developers.google.com/api-client-library/javascript/dev/dev_jscript#loading-the-client-library-and-the-api
  function loadAPIClientInterfaces() {
    gapi.client.load('youtube', 'v3', function() {
      gapi.client.load('youtubeAnalytics', 'v1', function() {
        // After both client interfaces load, use the Data API to request
        // information about the authenticated user's channel.
        getUserChannel();
      });
    });
  }

新增 channelId 變數和 getUserChannel 函式。函式會呼叫 YouTube Data API (v3) 並加入 mine 參數，代表該要求適用於目前已驗證使用者的頻道資訊。系統會將 channelId 傳送至 Analytics (分析) API，以識別您要擷取 Analytics (分析) 資料的頻道。


  // Keep track of the currently authenticated user's YouTube channel ID.
  var channelId;

  // Call the Data API to retrieve information about the currently
  // authenticated user's YouTube channel.
  function getUserChannel() {
    // Also see: https://developers.google.com/youtube/v3/docs/channels/list
    var request = gapi.client.youtube.channels.list({
      // Setting the "mine" request parameter's value to "true" indicates that
      // you want to retrieve the currently authenticated user's channel.
      mine: true,
      part: 'id,contentDetails'
    });

    request.execute(function(response) {
      if ('error' in response) {
        displayMessage(response.error.message);
      } else {
        // We need the channel's channel ID to make calls to the Analytics API.
        // The channel ID value has the form "UCdLFeWKpkLhkguiMZUp8lWA".
        channelId = response.items[0].id;
        // Retrieve the playlist ID that uniquely identifies the playlist of
        // videos uploaded to the authenticated user's channel. This value has
        // the form "UUdLFeWKpkLhkguiMZUp8lWA".
        var uploadsListId = response.items[0].contentDetails.relatedPlaylists.uploads;
        // Use the playlist ID to retrieve the list of uploaded videos.
        getPlaylistItems(uploadsListId);
      }
    });
  }

新增 getPlaylistItems 函式，以便擷取指定播放清單中的項目。在這種情況下，播放清單會列出上傳至使用者頻道的影片。(請注意，以下函式函式只會擷取該資訊提供中的前 50 個項目，因此您必須實作分頁才能擷取其他項目)。

擷取播放清單項目清單之後，函式會呼叫 getVideoMetadata() 函式。接著，這個函式會取得清單中與每部影片相關的中繼資料，並將每部影片加入使用者看到的清單。


  // Call the Data API to retrieve the items in a particular playlist. In this
  // example, we are retrieving a playlist of the currently authenticated user's
  // uploaded videos. By default, the list returns the most recent videos first.
  function getPlaylistItems(listId) {
    // See https://developers.google.com/youtube/v3/docs/playlistitems/list
    var request = gapi.client.youtube.playlistItems.list({
      playlistId: listId,
      part: 'snippet'
    });

    request.execute(function(response) {
      if ('error' in response) {
        displayMessage(response.error.message);
      } else {
        if ('items' in response) {
          // The jQuery.map() function iterates through all of the items in
          // the response and creates a new array that only contains the
          // specific property we're looking for: videoId.
          var videoIds = $.map(response.items, function(item) {
            return item.snippet.resourceId.videoId;
          });

          // Now that we know the IDs of all the videos in the uploads list,
          // we can retrieve information about each video.
          getVideoMetadata(videoIds);
        } else {
          displayMessage('There are no videos in your channel.');
        }
      }
    });
  }

  // Given an array of video IDs, this function obtains metadata about each
  // video and then uses that metadata to display a list of videos.
  function getVideoMetadata(videoIds) {
    // https://developers.google.com/youtube/v3/docs/videos/list
    var request = gapi.client.youtube.videos.list({
      // The 'id' property's value is a comma-separated string of video IDs.
      id: videoIds.join(','),
      part: 'id,snippet,statistics'
    });

    request.execute(function(response) {
      if ('error' in response) {
        displayMessage(response.error.message);
      } else {
        // Get the jQuery wrapper for the #video-list element before starting
        // the loop.
        var videoList = $('#video-list');
        $.each(response.items, function() {
          // Exclude videos that do not have any views, since those videos
          // will not have any interesting viewcount Analytics data.
          if (this.statistics.viewCount == 0) {
            return;
          }

          var title = this.snippet.title;
          var videoId = this.id;

          // Create a new <li> element that contains an <a> element.
          // Set the <a> element's text content to the video's title, and
          // add a click handler that will display Analytics data when invoked.
          var liElement = $('<li>');
          var aElement = $('<a>');
          // Setting the href value to '#' ensures that the browser renders the
          // <a> element as a clickable link.
          aElement.attr('href', '#');
          aElement.text(title);
          aElement.click(function() {
            displayVideoAnalytics(videoId);
          });

          // Call the jQuery.append() method to add the new <a> element to
          // the <li> element, and the <li> element to the parent
          // list, which is identified by the 'videoList' variable.
          liElement.append(aElement);
          videoList.append(liElement);
        });

        if (videoList.children().length == 0) {
          // Display a message if the channel does not have any viewed videos.
          displayMessage('Your channel does not have any videos that have been viewed.');
        }
      }
    });
  }

步驟 4：要求影片的 Analytics (分析) 資料

在這個步驟中，您必須修改範例應用程式，這樣當您按一下影片標題時，應用程式會呼叫 YouTube Analytics API，以擷取該影片的 Analytics (分析) 資料。如要進行此操作，請對範例應用程式進行下列變更：

新增變數，用來擷取擷取的 Analytics (分析) 報表資料的預設日期範圍。
```
  var ONE_MONTH_IN_MILLISECONDS = 1000 * 60 * 60 * 24 * 30;
```

新增程式碼，以便為日期物件建立 YYYY-MM-DD 字串，並將日期和月份的號碼換成兩位數：


  // This boilerplate code takes a Date object and returns a YYYY-MM-DD string.
  function formatDateString(date) {
    var yyyy = date.getFullYear().toString();
    var mm = padToTwoCharacters(date.getMonth() + 1);
    var dd = padToTwoCharacters(date.getDate());

    return yyyy + '-' + mm + '-' + dd;
  }

  // If number is a single digit, prepend a '0'. Otherwise, return the number
  //  as a string.
  function padToTwoCharacters(number) {
    if (number < 10) {
      return '0' + number;
    } else {
      return number.toString();
    }
  }

定義 displayVideoAnalytics 函式，以便擷取影片的 YouTube 數據分析資料。這個函式會在使用者點選清單中的影片時執行。輸出影片清單 (在步驟 3 中所定義) 的 getVideoMetadata 函式會定義點擊事件處理常式。


  // This function requests YouTube Analytics data for a video and displays
  // the results in a chart.
  function displayVideoAnalytics(videoId) {
    if (channelId) {
      // To use a different date range, modify the ONE_MONTH_IN_MILLISECONDS
      // variable to a different millisecond delta as desired.
      var today = new Date();
      var lastMonth = new Date(today.getTime() - ONE_MONTH_IN_MILLISECONDS);

      var request = gapi.client.youtubeAnalytics.reports.query({
        // The start-date and end-date parameters must be YYYY-MM-DD strings.
        'start-date': formatDateString(lastMonth),
        'end-date': formatDateString(today),
        // At this time, you need to explicitly specify channel==channelId.
        // See https://developers.google.com/youtube/analytics/v1/#ids
        ids: 'channel==' + channelId,
        dimensions: 'day',
        sort: 'day',
        // See https://developers.google.com/youtube/analytics/v1/available_reports
        // for details about the different filters and metrics you can request
        // if the "dimensions" parameter value is "day".
        metrics: 'views',
        filters: 'video==' + videoId
      });

      request.execute(function(response) {
        // This function is called regardless of whether the request succeeds.
        // The response contains YouTube Analytics data or an error message.
        if ('error' in response) {
          displayMessage(response.error.message);
        } else {
          displayChart(videoId, response);
        }
      });
    } else {
      // The currently authenticated user's channel ID is not available.
      displayMessage('The YouTube channel ID for the current user is not available.');
    }
  }

請參閱 API 說明文件的可用報表頁面，進一步瞭解可以擷取的資料，以及 metrics、dimensions 和 filters 參數的有效值組合。

步驟 5：在圖表中顯示 Analytics (分析) 資料

在這個步驟中，您必須新增 displayChart 函式，將 YouTube 數據分析資料傳送至 Google 視覺化 API。這樣 API 就會產生圖表資訊。

載入 Google 視覺化 API，以此在圖表中顯示您的資料。如需進一步瞭解圖表選項，請參閱 Visualization API 說明文件。
```
google.load('visualization', '1.0', {'packages': ['corechart']});
```

定義名為 displayChart 的新函式，該函式使用 Google 視覺化 API 動態產生顯示 Analytics (分析) 資料的圖表。


  // Call the Google Chart Tools API to generate a chart of Analytics data.
  function displayChart(videoId, response) {
    if ('rows' in response) {
      hideMessage();

      // The columnHeaders property contains an array of objects representing
      // each column's title -- e.g.: [{name:"day"},{name:"views"}]
      // We need these column titles as a simple array, so we call jQuery.map()
      // to get each element's "name" property and create a new array that only
      // contains those values.
      var columns = $.map(response.columnHeaders, function(item) {
        return item.name;
      });
      // The google.visualization.arrayToDataTable() function wants an array
      // of arrays. The first element is an array of column titles, calculated
      // above as "columns". The remaining elements are arrays that each
      // represent a row of data. Fortunately, response.rows is already in
      // this format, so it can just be concatenated.
      // See https://developers.google.com/chart/interactive/docs/datatables_dataviews#arraytodatatable
      var chartDataArray = [columns].concat(response.rows);
      var chartDataTable = google.visualization.arrayToDataTable(chartDataArray);

      var chart = new google.visualization.LineChart(document.getElementById('chart'));
      chart.draw(chartDataTable, {
        // Additional options can be set if desired as described at:
        // https://developers.google.com/chart/interactive/docs/reference#visdraw
        title: 'Views per Day of Video ' + videoId
      });
    } else {
      displayMessage('No data available for video ' + videoId);
    }
  }

查看完整的 index.js 檔案

下方的 index.js 檔案已包含上述步驟的所有變更。再次提醒您，您必須將YOUR_CLIENT_ID 字串換成已註冊應用程式的用戶端 ID。


(function() {
  // Retrieve your client ID from the Google API Console at
  // https://console.cloud.google.com/.
  var OAUTH2_CLIENT_ID = 'YOUR_CLIENT_ID';
  var OAUTH2_SCOPES = [
    'https://www.googleapis.com/auth/yt-analytics.readonly',
    'https://www.googleapis.com/auth/youtube.readonly'
  ];

  var ONE_MONTH_IN_MILLISECONDS = 1000 * 60 * 60 * 24 * 30;

  // Keep track of the currently authenticated user's YouTube channel ID.
  var channelId;

  // For information about the Google Chart Tools API, see:
  // https://developers.google.com/chart/interactive/docs/quick_start
  google.load('visualization', '1.0', {'packages': ['corechart']});

  // Upon loading, the Google APIs JS client automatically invokes this callback.
  // See https://developers.google.com/api-client-library/javascript/features/authentication 
  window.onJSClientLoad = function() {
    gapi.auth.init(function() {
      window.setTimeout(checkAuth, 1);
    });
  };

  // Attempt the immediate OAuth 2.0 client flow as soon as the page loads.
  // If the currently logged-in Google Account has previously authorized
  // the client specified as the OAUTH2_CLIENT_ID, then the authorization
  // succeeds with no user intervention. Otherwise, it fails and the
  // user interface that prompts for authorization needs to display.
  function checkAuth() {
    gapi.auth.authorize({
      client_id: OAUTH2_CLIENT_ID,
      scope: OAUTH2_SCOPES,
      immediate: true
    }, handleAuthResult);
  }

  // Handle the result of a gapi.auth.authorize() call.
  function handleAuthResult(authResult) {
    if (authResult) {
      // Authorization was successful. Hide authorization prompts and show
      // content that should be visible after authorization succeeds.
      $('.pre-auth').hide();
      $('.post-auth').show();

      loadAPIClientInterfaces();
    } else {
      // Authorization was unsuccessful. Show content related to prompting for
      // authorization and hide content that should be visible if authorization
      // succeeds.
      $('.post-auth').hide();
      $('.pre-auth').show();

      // Make the #login-link clickable. Attempt a non-immediate OAuth 2.0
      // client flow. The current function is called when that flow completes.
      $('#login-link').click(function() {
        gapi.auth.authorize({
          client_id: OAUTH2_CLIENT_ID,
          scope: OAUTH2_SCOPES,
          immediate: false
        }, handleAuthResult);
      });
    }
  }

  // Load the client interfaces for the YouTube Analytics and Data APIs, which
  // are required to use the Google APIs JS client. More info is available at
  // https://developers.google.com/api-client-library/javascript/dev/dev_jscript#loading-the-client-library-and-the-api
  function loadAPIClientInterfaces() {
    gapi.client.load('youtube', 'v3', function() {
      gapi.client.load('youtubeAnalytics', 'v1', function() {
        // After both client interfaces load, use the Data API to request
        // information about the authenticated user's channel.
        getUserChannel();
      });
    });
  }

  // Call the Data API to retrieve information about the currently
  // authenticated user's YouTube channel.
  function getUserChannel() {
    // Also see: https://developers.google.com/youtube/v3/docs/channels/list
    var request = gapi.client.youtube.channels.list({
      // Setting the "mine" request parameter's value to "true" indicates that
      // you want to retrieve the currently authenticated user's channel.
      mine: true,
      part: 'id,contentDetails'
    });

    request.execute(function(response) {
      if ('error' in response) {
        displayMessage(response.error.message);
      } else {
        // We need the channel's channel ID to make calls to the Analytics API.
        // The channel ID value has the form "UCdLFeWKpkLhkguiMZUp8lWA".
        channelId = response.items[0].id;
        // Retrieve the playlist ID that uniquely identifies the playlist of
        // videos uploaded to the authenticated user's channel. This value has
        // the form "UUdLFeWKpkLhkguiMZUp8lWA".
        var uploadsListId = response.items[0].contentDetails.relatedPlaylists.uploads;
        // Use the playlist ID to retrieve the list of uploaded videos.
        getPlaylistItems(uploadsListId);
      }
    });
  }

  // Call the Data API to retrieve the items in a particular playlist. In this
  // example, we are retrieving a playlist of the currently authenticated user's
  // uploaded videos. By default, the list returns the most recent videos first.
  function getPlaylistItems(listId) {
    // See https://developers.google.com/youtube/v3/docs/playlistitems/list
    var request = gapi.client.youtube.playlistItems.list({
      playlistId: listId,
      part: 'snippet'
    });

    request.execute(function(response) {
      if ('error' in response) {
        displayMessage(response.error.message);
      } else {
        if ('items' in response) {
          // The jQuery.map() function iterates through all of the items in
          // the response and creates a new array that only contains the
          // specific property we're looking for: videoId.
          var videoIds = $.map(response.items, function(item) {
            return item.snippet.resourceId.videoId;
          });

          // Now that we know the IDs of all the videos in the uploads list,
          // we can retrieve information about each video.
          getVideoMetadata(videoIds);
        } else {
          displayMessage('There are no videos in your channel.');
        }
      }
    });
  }

  // Given an array of video IDs, this function obtains metadata about each
  // video and then uses that metadata to display a list of videos.
  function getVideoMetadata(videoIds) {
    // https://developers.google.com/youtube/v3/docs/videos/list
    var request = gapi.client.youtube.videos.list({
      // The 'id' property's value is a comma-separated string of video IDs.
      id: videoIds.join(','),
      part: 'id,snippet,statistics'
    });

    request.execute(function(response) {
      if ('error' in response) {
        displayMessage(response.error.message);
      } else {
        // Get the jQuery wrapper for the #video-list element before starting
        // the loop.
        var videoList = $('#video-list');
        $.each(response.items, function() {
          // Exclude videos that do not have any views, since those videos
          // will not have any interesting viewcount Analytics data.
          if (this.statistics.viewCount == 0) {
            return;
          }

          var title = this.snippet.title;
          var videoId = this.id;

          // Create a new <li> element that contains an <a> element.
          // Set the <a> element's text content to the video's title, and
          // add a click handler that will display Analytics data when invoked.
          var liElement = $('<li>');
          var aElement = $('<a>');
          // Setting the href value to '#' ensures that the browser renders the
          // <a> element as a clickable link.
          aElement.attr('href', '#');
          aElement.text(title);
          aElement.click(function() {
            displayVideoAnalytics(videoId);
          });

          // Call the jQuery.append() method to add the new <a> element to
          // the <li> element, and the <li> element to the parent
          // list, which is identified by the 'videoList' variable.
          liElement.append(aElement);
          videoList.append(liElement);
        });

        if (videoList.children().length == 0) {
          // Display a message if the channel does not have any viewed videos.
          displayMessage('Your channel does not have any videos that have been viewed.');
        }
      }
    });
  }

  // This function requests YouTube Analytics data for a video and displays
  // the results in a chart.
  function displayVideoAnalytics(videoId) {
    if (channelId) {
      // To use a different date range, modify the ONE_MONTH_IN_MILLISECONDS
      // variable to a different millisecond delta as desired.
      var today = new Date();
      var lastMonth = new Date(today.getTime() - ONE_MONTH_IN_MILLISECONDS);

      var request = gapi.client.youtubeAnalytics.reports.query({
        // The start-date and end-date parameters must be YYYY-MM-DD strings.
        'start-date': formatDateString(lastMonth),
        'end-date': formatDateString(today),
        // At this time, you need to explicitly specify channel==channelId.
        // See https://developers.google.com/youtube/analytics/v1/#ids
        ids: 'channel==' + channelId,
        dimensions: 'day',
        sort: 'day',
        // See https://developers.google.com/youtube/analytics/v1/available_reports
        // for details about the different filters and metrics you can request
        // if the "dimensions" parameter value is "day".
        metrics: 'views',
        filters: 'video==' + videoId
      });

      request.execute(function(response) {
        // This function is called regardless of whether the request succeeds.
        // The response contains YouTube Analytics data or an error message.
        if ('error' in response) {
          displayMessage(response.error.message);
        } else {
          displayChart(videoId, response);
        }
      });
    } else {
      // The currently authenticated user's channel ID is not available.
      displayMessage('The YouTube channel ID for the current user is not available.');
    }
  }

  // This boilerplate code takes a Date object and returns a YYYY-MM-DD string.
  function formatDateString(date) {
    var yyyy = date.getFullYear().toString();
    var mm = padToTwoCharacters(date.getMonth() + 1);
    var dd = padToTwoCharacters(date.getDate());

    return yyyy + '-' + mm + '-' + dd;
  }

  // If number is a single digit, prepend a '0'. Otherwise, return the number
  //  as a string.
  function padToTwoCharacters(number) {
    if (number < 10) {
      return '0' + number;
    } else {
      return number.toString();
    }
  }

  // Call the Google Chart Tools API to generate a chart of Analytics data.
  function displayChart(videoId, response) {
    if ('rows' in response) {
      hideMessage();

      // The columnHeaders property contains an array of objects representing
      // each column's title -- e.g.: [{name:"day"},{name:"views"}]
      // We need these column titles as a simple array, so we call jQuery.map()
      // to get each element's "name" property and create a new array that only
      // contains those values.
      var columns = $.map(response.columnHeaders, function(item) {
        return item.name;
      });
      // The google.visualization.arrayToDataTable() function wants an array
      // of arrays. The first element is an array of column titles, calculated
      // above as "columns". The remaining elements are arrays that each
      // represent a row of data. Fortunately, response.rows is already in
      // this format, so it can just be concatenated.
      // See https://developers.google.com/chart/interactive/docs/datatables_dataviews#arraytodatatable
      var chartDataArray = [columns].concat(response.rows);
      var chartDataTable = google.visualization.arrayToDataTable(chartDataArray);

      var chart = new google.visualization.LineChart(document.getElementById('chart'));
      chart.draw(chartDataTable, {
        // Additional options can be set if desired as described at:
        // https://developers.google.com/chart/interactive/docs/reference#visdraw
        title: 'Views per Day of Video ' + videoId
      });
    } else {
      displayMessage('No data available for video ' + videoId);
    }
  }

  // This helper method displays a message on the page.
  function displayMessage(message) {
    $('#message').text(message).show();
  }

  // This helper method hides a previously displayed message on the page.
  function hideMessage() {
    $('#message').hide();
  }
})();