I know this is an old question, but much of the information in the accepted answer has been deprecated by Google since then. DocsList is gone, as are the clear() and append() methods on a file object.
I use the following function to create or overwrite a file:
// Creates or replaces an existing file
function updateFile (folder, filename, data) {
try {
// filename is unique, so we can get first element of iterator
var file = folder.getFilesByName(filename).next()
file.setContent(data)
} catch(e) {
folder.createFile(filename, data)
}
}
For reference, here's some code for doing the same for a folder. It assumes we're operating in the parent folder of the current sheet and want a folder
object for a new or existing folder there.
// Get folder under current folder. Create it if it does not exist
function getOrCreateFolder(csvFolderName) {
var thisFileId = SpreadsheetApp.getActive().getId();
var thisFile = DriveApp.getFileById(thisFileId);
var parentFolder = thisFile.getParents().next();
try {
// csvFolderName is unique, so we can get first element of iterator
var folder = parentFolder.getFoldersByName(csvFolderName).next();
// asking for the folder's name ensures we get an error if the
// folder doesn't exist. I've found I don't necessarily catch
// an error from getFoldersByName if csvFolderName doesn't exist.
fname = folder.getName()
} catch(e) {
var folder = parentFolder.createFolder(csvFolderName);
}
return folder
}