[{name:"xyz",age:"100",arr:["a","ab"]},{name:"abc",age:"123",arr:["b","ab"]}]

db.collection.aggregate([{$addFields:{data:{$objectToArray:{"a":"data a","ab":"data a","b":"data b","c":"data c"}},}},{$addFields:{data:{$arrayElemAt:["$data.v",{$indexOfArray:["$data.k",{$arrayElemAt:["$arr",0]}]}]}}}])