how to add value in array in mongodb code example

Example 1: mongodb add multiple element to array

db.students.update(
   { name: "joe" },
   { $push: { scores: { $each: [ 90, 92, 85 ] } } }
)

Example 2: mongodb push to arry in update

db.collection.update({_id:xx}, {$pop:{letters : -1}})