[{name:"Object 1",fruitList:["apple","pear","orange","grape"]},{name:"Object 2",fruitList:["melon","pear","apple","kiwi"]}]

db.collection.aggregate([{$addFields:{appleIndex:{$indexOfArray:["$fruitList","apple"]},pearIndex:{$indexOfArray:["$fruitList","pear"]},indexDifference:{$subtract:[{$indexOfArray:["$fruitList","apple"]},{$indexOfArray:["$fruitList","pear"]}]}}}])