How to get the index of an array element in older versions on MongoDB?

To get the index of an array element in older versions of MongoDB, use the $indexOfArray aggregation operator. This operator returns the position (0-based index) of the first occurrence of a specified value in an array, or -1 if the value is not found.

Syntax

db.collection.aggregate([
    {
        $project: {
            fieldName: { $indexOfArray: ["$arrayField", "searchValue"] }
        }
    }
]);

Sample Data

db.demo65.insertMany([
    { "ListOfValues": [10, 20, 30] },
    { "ListOfValues": [50, 60, 70, 100] },
    { "ListOfValues": [30, 40, 89, 91, 98] }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e28990ecfb11e5c34d89938"),
        ObjectId("5e28991ecfb11e5c34d89939"),
        ObjectId("5e28992bcfb11e5c34d8993a")
    ]
}

Display Sample Data

db.demo65.find();

{ "_id": ObjectId("5e28990ecfb11e5c34d89938"), "ListOfValues": [10, 20, 30] }
{ "_id": ObjectId("5e28991ecfb11e5c34d89939"), "ListOfValues": [50, 60, 70, 100] }
{ "_id": ObjectId("5e28992bcfb11e5c34d8993a"), "ListOfValues": [30, 40, 89, 91, 98] }

Example: Find Index of Value 30

db.demo65.aggregate([
    {
        $project: {
            IndexOfAnArrayElement: { $indexOfArray: ["$ListOfValues", 30] }
        }
    }
]);

{ "_id": ObjectId("5e28990ecfb11e5c34d89938"), "IndexOfAnArrayElement": 2 }
{ "_id": ObjectId("5e28991ecfb11e5c34d89939"), "IndexOfAnArrayElement": -1 }
{ "_id": ObjectId("5e28992bcfb11e5c34d8993a"), "IndexOfAnArrayElement": 0 }

Key Points

• $indexOfArray returns 0-based index of the first match.
• Returns -1 when the value is not found in the array.
• Works with $project stage in aggregation pipeline.

Conclusion

The $indexOfArray operator provides an efficient way to find element positions in arrays during aggregation. It returns the zero-based index of the first match or -1 if the value doesn't exist in the array.