How to fetch the ID of a Google Spread Sheet via Google Apps Script?

The ID (key) of the spreadsheet is in the URL (the part between key= and #gid=). You can retrieve it with GAS using something like:

function getId() {
  Browser.msgBox('Spreadsheet key: ' + SpreadsheetApp.getActiveSpreadsheet().getId());
}

Note, often you will get a totally different string with each method, but they both should work the same.

Remember that the getId() function from a Spreadsheet object returns a different id compared with getId() from File object, even if the File (that is managed from DriveApp) is the same spreadsheet.

Anyhow, if you open a file from DriveApp using the id provided by the Spreadsheet, you will obtain the correct File object, returning - with getId() - the "File" id, which is different from the one that you used to open the File.

It seems confusing, but it works so. I had some issues in some scripts coming from this "double" id for the same thing.