[{"array":[{"val1":"a"},{"val1":"b"}]}]

db.collection.aggregate({$addFields:{array:{$map:{input:"$array",as:"a",in:{$cond:[{$eq:["$val1","a"]},{"$mergeObjects":["$$a",{"Isval1A":true}]},{"$mergeObjects":["$$a",{"Isval1A":false}]}]}}}}})