function markVideos() {
  let ss = SpreadsheetApp.getActiveSpreadsheet();
  let sheets = SpreadsheetApp.getActiveSpreadsheet().getSheets();
  
  // Runs through process for each tab in Spreadsheet.
  sheets.forEach(function(dataSheet) {
    let tabName = dataSheet.getName();
    let range = dataSheet.getDataRange();
    let numRows = range.getNumRows();
    let rows = range.getValues();
    let headerRow = rows[0];
    
    // Finds the column indices.
    let videoColumnIdx = headerRow.indexOf(COLUMN_NAME.VIDEO);
    let titleColumnIdx = headerRow.indexOf(COLUMN_NAME.TITLE);
    
    // Creates empty array to collect data for email table.
    let emailContent = [];
    
    // Processes each row in spreadsheet.
    for (let i = 1; i < numRows; ++i) {
      let row = rows[i];
      // Extracts video ID.
      let videoId = extractVideoIdFromUrl(row[videoColumnIdx])
      // Processes each row that contains a video ID.
      if(!videoId) { 
        continue;
      }
      // Calls getVideoDetails function and extracts target data for the video.
      let detailsResponse = getVideoDetails(videoId);
      let title = detailsResponse.items[0].snippet.title;
      let publishDate = detailsResponse.items[0].snippet.publishedAt;
      let publishDateFormatted = new Date(publishDate);
      let views = detailsResponse.items[0].statistics.viewCount;
      let likes = detailsResponse.items[0].statistics.likeCount;
      let comments = detailsResponse.items[0].statistics.commentCount;
      let channel = detailsResponse.items[0].snippet.channelTitle;
      
      // Collects title, publish date, channel, views, comments, likes details and pastes into tab.
      let detailsRow = [title,publishDateFormatted,channel,views,comments,likes];
      dataSheet.getRange(i+1,titleColumnIdx+1,1,6).setValues([detailsRow]);
      
      // Determines if new count of comments/replies is greater than old count of comments/replies.
      let addlCommentCount = comments - row[titleColumnIdx+4];
      
      // Adds video title, link, and additional comment count to table if new counts > old counts.
      if (addlCommentCount > 0) {
        let emailRow = [title,row[videoColumnIdx],addlCommentCount]
        emailContent.push(emailRow);
      }
    }
    // Sends notification email if Content is not empty.
    if (emailContent.length > 0 && EMAIL_ON == 'Y') {
      sendEmailNotificationTemplate(emailContent, tabName);
    }
  });
}
Gets YouTube video details and statistics for all
video URLs listed in 'Video Link' column in each
sheet. Sends email summary, based on preferences above, 
when videos have new comments or replies.