folder.getFiles is not a function error in Trigger - google-apps-script

Question

When I run my function in Apps Script its working fine but when I add it in a trigger this error always occurs in first run.
TypeError: folder.getFiles is not a function
This is the code. I use this code to get all files in all folders and subfolders.
var level=0;
function getFnF(folder) {
var folder= folder || DriveApp.getFolderById('root folder ID');
var ss=SpreadsheetApp.openById('google sheet ID');
var sh=ss.getSheetByName('List');
var files=folder.getFiles();
while(files.hasNext()) {
var file=files.next();
var fileid = file.getId();
var TimeStamp = Utilities.formatDate(new Date(), "GMT+8", "MMMM dd, yyyy HH:mm:ss")
var filesize = file.getSize()/ 1024 / 1024;
var lastrow = sh.getLastRow();
var filename =file.getName();
sh.appendRow([TimeStamp,lastrow,filename,file.getName(),filesize.toFixed(2),fileid);
Logger.log('Item added in list: ['+ lastrow+"] " + filename);
}
var subfolders=folder.getFolders()
while(subfolders.hasNext()) {
var subfolder=subfolders.next();
//subfolders only
level++;
getFnF(subfolder);
}
level--;
}
I hope you can help me.

Answer 1

From When I run my function in apps script its working fine but when I add it in a trigger this error always occurs in first run., I'm not sure about the kind of trigger. But when the function is run by a trigger, the event object is given to the 1st argument. So, in this case, how about the following modification?
Modified script:
var level = 0;
function getFnF(e, folder = DriveApp.getFolderById('root folder ID')) {
var ss = SpreadsheetApp.openById('google sheet ID');
var sh = ss.getSheetByName('Sheet1');
var files = folder.getFiles();
while (files.hasNext()) {
var file = files.next();
var fileid = file.getId();
var TimeStamp = Utilities.formatDate(new Date(), "GMT+8", "MMMM dd, yyyy HH:mm:ss")
var filesize = file.getSize() / 1024 / 1024;
var lastrow = sh.getLastRow();
var filename = file.getName();
sh.appendRow([TimeStamp, lastrow, filename, file.getName(), filesize.toFixed(2), fileid]);
Logger.log('Item added in list: [' + lastrow + "] " + filename);
}
var subfolders = folder.getFolders()
while (subfolders.hasNext()) {
var subfolder = subfolders.next();
//subfolders only
level++;
getFnF(e, subfolder);
}
level--;
}
By this modification, when the function getFnF by a trigger, the function is run as the initial value of folder of DriveApp.getFolderById('root folder ID').
By the way, when you want to run this function from another function instead of the trigger, please call this as getFnF("dummy", folder).