[{_id:1,name:"a",info:[]},{_id:2,name:"b",info:[{infoID:100,infoData:"my info"}]},{_id:3,name:"c",info:[{infoID:200,infoData:"some info 200"},{infoID:300,infoData:"some info 300"}]}]

db.collection.aggregate([{$addFields:{infoData100:{$cond:[{$in:[100,"$info.infoID"]},{$let:{vars:{data:{$arrayElemAt:[{$filter:{input:"$info",cond:{$eq:["$$this.infoID",100]}}},0]}},in:"$$data.infoData"}},null]}}}])