push_slice.js

> db.tracking.insert({
      "_id": "44444", 
      "recentItems": ['Apples', 'Bananas', 'Cherries']
  })
> db.tracking.update(
      { "_id": "44444" },
      { 
          $push: {
               "recentItems": {
                  $each: ['Dates'],
                  $slice: -3,
              }
          }
      }
  )
// Check the results:
> db.tracking.find({ "_id": "44444" })
{ "recentItems" : [ "Bananas", "Cherries", "Dates" ] }