/**
 * Writes a CSV file to a Drive folder (by id) containing filenames and Google Drive download urls (File.webContentLink)
 * for all files in a specified folder (by id) of a Shared Drive (by id).
*/
function ExportFilesFromSharedDriveToCSV() {
  // id of Shared Drive (go to Shared Drive, copy-paste id from browser URL)
  var sharedDriveId = "0AFR1j1Oo3fnlUk9PVA";
  
  // id of folder in Shared Drive to search (go to folder, copy-paste if from browser URL)
  var folderIdToSearch = "1Ua6mIE8kL4Q1r-LEEVmAlo-oyrsY2gg9";
  
  // id of folder to save the CSV file
  var folderIdToSave = "1HOE7yqsk4bQDC6d7CwNoR7wAbeIN5Csi"
  
  // list of File objects from a folder. See: 
  //    https://developers.google.com/drive/api/v3/reference/files#resource
  //    https://developers.google.com/drive/api/v3/search-files
  var fileItems = GetFilesFromSharedDrive(sharedDriveId, folderIdToSearch);
  
  // join filename and download url into csv, save to a Drive folder
  var csvText = MakeCSVFromFileItems(fileItems);
  
  // write multi-line comma-separated string variable to specified folder (by id) with specified name
  var file = WriteVariableToFile(csvText, folderIdToSave, 'output.csv');
}




/**
 * Creates a multi-line string of CSV text from list of Files.
 *
 * @param {Files resource} fileItems collection of File objects. See: https://developers.google.com/drive/api/v3/reference/files#resource
 *
 * @return {String} csvText The multi-line string to write to a CSV file.
*/
function MakeCSVFromFileItems(fileItems) {
  // make header row of CSV
  var list = ['title, downloadUrl'];
  
  // format list of files into list of comma-separated strings, for filename and url
  for (var i = 0; i < fileItems.length; i++) {
    // ignore files without a webContentLink (download url)
    if (!fileItems[i].webContentLink) {continue;}
    
    // many other File attributes are available. using `title` (filename) and `webContentLink` (download url)
    list.push(Utilities.formatString('%s,%s', fileItems[i].title, fileItems[i].webContentLink));
  }
  
  // return a multi-line string from list of values, using newline characters
  return list.join('\n')
}




/**
 * Searches for files in a Shared Drive (by id) whose parent folder matches `folderIdToSearch`.
 *
 * @param {String} sharedDriveId Id of Shared Drive (navigate to root in Drive, copy id from URL in browser).
 * @param {String} folderIdToSearch Id of folder in Shared Drive to search (navigate to folder in Drive, copy id from URL in browser).
 *
 * @return {Array} fileItems List of File resources each with attributes like filename and download url.
*/
function GetFilesFromSharedDrive(sharedDriveId, folderIdToSearch) {
  // search a folder within a Shared Drive and get all files within it
  var folderId = folderIdToSearch || "1Ua6mIE8kL4Q1r-LEEVmAlo-oyrsY2gg9";
  var sharedDriveId = sharedDriveId || "0AFR1j1Oo3fnlUk9PVA";
  var result = Drive.Files.list({
    "corpora": "teamDrive",
    "includeTeamDriveItems": true,
    "orderBy": "folder",
    "q": Utilities.formatString('\'%s\' in parents',folderId),  // must wrap value with ' '
    "supportsTeamDrives": true,
    "driveId": sharedDriveId
  });
  
  // return the data as object (JSON parse string into object)
  return JSON.parse(result).items;
}




/**
 * Saves an Apps Script variable to a file (by name) in a specified folder (by id).
 *
 * @param {Object} variable Any variable allowed, but String used for this case.
 * @param {String} folderId Id of folder in Google Drive to save the new file.
 * @param {String} filename String name of the new file to save.
 * 
 * @return {File} file The resulting newly-created file.
*/
function WriteVariableToFile(variable, folderId, filename) {
  var folderId = folderId || "1HOE7yqsk4bQDC6d7CwNoR7wAbeIN5Csi";
  var folder = DriveApp.getFolderById(folderId);
  var files = folder.getFilesByName(filename);
  
  // guarantees there is only one file with that name in the folder.
  while (files.hasNext()) {
    files.next().setTrashed(true);  
  }
  
  return folder.createFile(filename, variable);
}