[{x:["https://images.freeimages.com/images/large-previews/e51/tokyo05-2-1447803.jpg","https://images.freeimages.com/images/large-previews/aae/lomo-spider-1386711.jpg","https://images.freeimages.com/images/large-previews/383/the-home-of-the-candle-1-1425911.jpg"]}]

db.collection.update({},[{$addFields:{x:{"$map":{"input":"$x","as":"y","in":{url:"$$y",index:{$indexOfArray:["$x","$$y"]}}}}}}],{multi:true})