Retrieving Spreadsheet ID from Range using Google Apps Script · GitHub

var id = "123456789abcdefg";
var sheet = "Sheet";
var cells = "a1:b10";
var range = SpreadsheetApp.openById(id).getSheetByName(sheet).getRange(cells);

var id = range.getSheet().getParent().getId();

>>> id ---> 123456789abcdefg