If there are multiple documents in the collection that match your conditions, you can update only suitable one by adding { weight: { $type: 4 } } to your conditions.
conditions
{ weight: { $type: 4 } }
Otherwise your application's schema doesn't match data in the database.
This might work.
//Schema
weight: [Number]
http://mongoosejs.com/docs/schematypes.html
//Or this way too if pushing objects into array
weight: [{ weight: { type: Number }}]
//Then in API
var update = { $push: {"weight": { "weight": req.body.weight }}};