How can i get item id

I have used the following code to get the item (As user insert only once )

wixData.query("user").eq("_ownerId", "ffdkj9c2-df8g-f9ke-lk98-4kjhfr89keedb")
  .find()
  .then( (results) => {
    
      let firstItem = results.items[0]; //see item below
   })
/*  firstItem is:
 *
 *  {
 *    "_id":          "00001",
 *    "_owner":       "ffdkj9c2-df8g-f9ke-lk98-4kjhfr89keedb",
 *    "_createdDate": "2017-05-24T12:33:18.938Z",
 *    "_updatedDate": "2017-05-24T12:33:18.938Z",
 *    "title":        "Mr.",
 *    "first_name":   "John",
 *    "last_name":    "Doe"
 *  }
 */

how can i get the "_id " of this item

firstItem._id

Thanks a lot it worked