mongo $push if array, if not set code example

Example 1: mongodb push to arry in update

db.collection.update({_id:xx}, {$push:{letters : {$each:['first one'], $position:0}}})

Example 2: mongodb add to array if not exists

db.tags.update(
    {name: 'sport'},
    {$addToSet: { videoIDs: "34f54e34c" } }
);