how to add new data to an existing document in mongodb code example

Example: how to add a new propety into all documents in mongodb

db.users.update({}, { "$set" : { "age": 30 }}, false,true)
// users: collection name, age: new property
//false it's upsert argument, it tells mongo to not insert a new document when no match is found
// true it's multi argument, it tells mongo to update multiple documents that meet the query criteria