Get index of given element in array field in MongoDB?

To get the index of a given element in an array field in MongoDB, use the $indexOfArray operator within an aggregation pipeline. This operator returns the zero-based index of the first occurrence of the specified element in the array.

Syntax

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

Sample Data

db.getIndexDemo.insertOne({
    "InstructorName": "Chris",
    "InstructorSubject": ["MongoDB", "MySQL", "Java", "C++"]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5cbd5251de8cc557214c0df8")
}

Display all documents from the collection ?

db.getIndexDemo.find().pretty();

{
    "_id": ObjectId("5cbd5251de8cc557214c0df8"),
    "InstructorName": "Chris",
    "InstructorSubject": [
        "MongoDB",
        "MySQL", 
        "Java",
        "C++"
    ]
}

Example 1: Find Index of "MongoDB"

db.getIndexDemo.aggregate([
    {
        $project: {
            "matchedIndex": {
                $indexOfArray: ["$InstructorSubject", "MongoDB"]
            }
        }
    }
]);

{ "_id": ObjectId("5cbd5251de8cc557214c0df8"), "matchedIndex": 0 }

Example 2: Find Index of "C++"

db.getIndexDemo.aggregate([
    {
        $project: {
            "matchedIndex": {
                $indexOfArray: ["$InstructorSubject", "C++"]
            }
        }
    }
]);

{ "_id": ObjectId("5cbd5251de8cc557214c0df8"), "matchedIndex": 3 }

Key Points

• $indexOfArray returns -1 if the element is not found in the array
• Array indexing starts from 0, where the first element has index 0
• The operator returns the index of the first occurrence of the element

Conclusion

The $indexOfArray operator efficiently finds the zero-based position of elements in array fields. Use it within aggregation pipelines to locate specific values and perform index-based operations on array data.