Last modified date in Google App Script GAS sheets - google-apps-script

Question

I am trying to get the last modified date of a sheet from a GAS add on which I am developing.
My current idea is to get the Drive revision list and then take the last value. This seems a bit overkill for just getting the last modified, I am also worried that this will break if the number of revisions exceeds 1000 as per this link suggests.
https://developers.google.com/drive/api/v3/reference/revisions/list
Ideally I would like to know the range which has changed too, but I do not think this is possible.
I cannot use the onEdit event because I would like to track edits made by users who have not installed the add-on.
var fileId = SpreadsheetApp.getActiveSpreadsheet().getId();
var revisions = Drive.Revisions.list(fileId);
var revisionLength = revisions.items.length;
if(revisionLength > 0){
var revision = revisions.items[revisionLength-1];
var date = new Date(revision.modifiedDate);
Logger.log(date.toString());
}

Answer 1

I believe you can do that as follow
var lastUpdated = DriveApp.getFileById(fileId).getLastUpdated();
See function reference

Answer 2

Given that you need users of your add-on to have access to revision information from non-add-on users, the Drive revision list is precisely what you need. Happily, you can get the content of revisions, so if you wish you can compute diffs. I don't know what your data looks like, so that might be easy or nigh-impossible.
Aside: to your point about more than 1000 revisions, if there are more than 1000 (or whatever your page size is) revisions, you'll get a nextPageToken like so:
{
"kind": "drive#revisionList",
"nextPageToken": "BHNMJKHJKHKVJHyugaiohasdzT1JyUmlQWG10RUJ1emx1S2xNDg4EgQzMzY1GAI=",
"revisions": [
...
]
}
If you see that you'll need to list revisions again, providing that token.
Anyway, when you list revisions, each revision will look something like this:
{
"kind": "drive#revision",
"etag": "\"som3-e-tAg\"",
"id": "3365",
"selfLink": "https://www.googleapis.com/drive/v2/files/dummydummydummy/revisions/3365",
"mimeType": "application/vnd.google-apps.spreadsheet",
"modifiedDate": "2018-10-19T19:05:41.762Z",
"published": false,
"exportLinks": {
"application/x-vnd.oasis.opendocument.spreadsheet": "https://docs.google.com/spreadsheets/export?id=dummydummydummy&revision=3365&exportFormat=ods",
"text/tab-separated-values": "https://docs.google.com/spreadsheets/export?id=dummydummydummy&revision=3365&exportFormat=tsv",
"application/pdf": "https://docs.google.com/spreadsheets/export?id=dummydummydummy&revision=3365&exportFormat=pdf",
"application/vnd.openxmlformats-officedocument.spreadsheetml.sheet": "https://docs.google.com/spreadsheets/export?id=dummydummydummy&revision=3365&exportFormat=xlsx",
"text/csv": "https://docs.google.com/spreadsheets/export?id=dummydummydummy&revision=3365&exportFormat=csv",
"application/zip": "https://docs.google.com/spreadsheets/export?id=dummydummydummy&revision=3365&exportFormat=zip",
"application/vnd.oasis.opendocument.spreadsheet": "https://docs.google.com/spreadsheets/export?id=dummydummydummy&revision=3365&exportFormat=ods"
},
"lastModifyingUserName": "Joe User",
"lastModifyingUser": {
"kind": "drive#user",
"displayName": "Joe User",
"picture": {
"url": "https://lh3.googleusercontent.com/-asdfsadf/AAAAAAAAAAI/AAAAAAAAFOk/OIPUYOIUGO/s64/photo.jpg"
},
"isAuthenticatedUser": true,
"permissionId": "123456789",
"emailAddress": "user#gmail.com"
}
}
Provided your data isn't insanely complex or large, you could fetch the target the text/csv export link for the revisions you wish to compare, and then do that comparison in Apps Script.
That might look something like:
var fileId = SpreadsheetApp.getActiveSpreadsheet().getId();
var revisions = Drive.Revisions.list(fileId);
var revisionLength = revisions.items.length;
if(revisionLength > 1) { // something to compare!
var revision = revisions.items[revisionLength-1];
var newContent = UrlFetchApp.fetch(revision.exportLinks["text/csv"]).getContent();
newContent = Utilities.parseCsv(newContent);
var oldRevision = revisions.items[revisionLength-2];
var oldContent = UrlFetchApp.fetch(oldRevision.exportLinks["text/csv"]).getContent();
oldContent = Utilities.parseCsv(oldContent);
# TODO check they're the same size!
# where do they differ?
for (var row = 0; row < newContent.length; row++) {
for (var col = 0; col < newContent[0].length; col++) {
if (newContent[row][col] != oldContent[row][col]) {
Logger.log('Change on row ' + (row + 1) + ' column ' + (col + 1));
}
}
# when did it change?
var date = new Date(revision.modifiedDate);
Logger.log(date.toString());
}