Database

[{"_id":"1","authorName":"...","articles":[{"articleTitle":"...","views":4},{"articleTitle":"...","views":2},{"articleTitle":"...","views":10}]}]

Query

db.collection.aggregate([{"$set":{"totalArticleViews":{"$reduce":{"input":"$articles","initialValue":0,"in":{"$let":{"vars":{"tviews":"$$value","article":"$$this"},"in":{"$add":["$$tviews","$$article.views"]}}}}}}}])

Result