Get Google Sheet by ID?

You can use something like this :

function getSheetById(id) {
  return SpreadsheetApp.getActive().getSheets().filter(
    function(s) {return s.getSheetId() === id;}
  )[0];
}

var sheet = getSheetById(123456789);

And then to find the sheet ID to use for the active sheet, run this and check the Logs or use the debugger.

function getActiveSheetId(){
  var id  = SpreadsheetApp.getActiveSheet().getSheetId();
  Logger.log(id.toString());
  return id;
}