Database
[{_id:1,items:[{id:"6614005475802af9ae05b6b2",title:"title",createdAt:"2024-04-08T14:33:56.734Z"},{id:"6613fda792ffb86a14d266ea",title:"testing",createdAt:"2024-04-08T14:22:30.880Z"},{id:"6613fe3675802af9ae05b6a2",title:"cool stuff",createdAt:"2024-04-08T14:24:54.580Z"}]}]
Query
db.collection.update({},[{$set:{"items":{$map:{input:"$items",as:"item",in:{$mergeObjects:["$$item",{"position":{$indexOfArray:["$items","$$item"]}}]}}}}}],{multi:true})