MongoDB $arrayElemAt Operator (original) (raw)

Last Updated : 15 Apr, 2026

The $arrayElemAt operator retrieves a specific element from an array by index and is commonly used in aggregation pipelines to extract targeted values efficiently.

• Returns an element at a specified position in an array.
• Useful for accessing first, last, or any indexed value.
• Works within aggregation stages for data transformation.
• Useful for directly accessing specific elements in an array without needing additional processing logic.

Syntax

{ $arrayElemAt: [ , ] }

• ****:** An array or an expression that resolves to an array.
• ****:** The index of the element to retrieve. It can be a positive or negative integer.

Features of $arrayElemAt Operator

Here are some features:

• **Positive Index: Retrieves the element starting from the beginning of the array (0-based index).
• **Negative Index: Retrieves elements counting backward from the end of the array.
• **Out-of-Bounds Index: Returns null if the index is outside the array range.

Examples of MongoDB $arrayElemAt

In the following examples, we are working with:

**Database: GeeksforGeeks
**Collection: arrayExample
**Document: Three documents that contain the details in the form of field-value pairs.

Example 1: Retrieving First and Last Items

Find the elements of the array(i.e., the value of fruits field) on the specified index using $arrayElemAt operator.

db.arrayExample.aggregate([
{ $match: { name: "Bruno" } },
{
$project: {
firstItem: { arrayElemAt:["arrayElemAt: ["arrayElemAt:["fruits", 0] },
lastItem: { arrayElemAt:["arrayElemAt: ["arrayElemAt:["fruits", -1] }
}
}
])

**Output:

• The $arrayElemAt operator extracts the first and last elements of the fruits array.
• The element at index 0 is "Mango", and the element at index -1 (last) is "Apple".

Example 2: Using $arrayElemAt Operator in the Embedded Document

Find the elements from the outdoorGames array, which is inside the embedded document favGame, for a document with name: "Pika" using $arrayElemAt operator.

db.arrayExample.aggregate([
{ $match: { name: "Pika" } },
{
$project: {
firstItem: { arrayElemAt:["arrayElemAt: ["arrayElemAt:["favGame.outdoorGames", 0] },
item: { arrayElemAt:["arrayElemAt: ["arrayElemAt:["favGame.outdoorGames", 2] }
}
}
])

**Output:

• Extracts specific elements from favGame.outdoorGames using $arrayElemAt.
• Returns "Cricket" (index 0) and "Hockey" (index 2).

Best Practices for Using MongoDB $arrayElemAt

When working with the $arrayElemAt operator, it's important to follow best practices to ensure efficiency and correctness:

• **Ensure Valid Index Values: Verify indices before access; out-of-range indices return null (use $size to compute bounds).
• **Use Within Aggregation Pipeline: Apply arrayElemAtinstageslikearrayElemAt in stages like arrayElemAtinstageslikeproject, addFields,andaddFields, and addFields,andgroup for transformations.
• **Combine with Other Operators: Pair with ifNull,ifNull, ifNull,cond, and $size to handle missing or empty arrays safely.
• **Optimize Performance: Use judiciously in complex MongoDB queries to avoid overhead on large datasets.

Use Cases for MongoDB $arrayElemAt

Here are some use cases discussed:

• Retrieve the first or last element of an array using index 0 or -1.
• Access elements at specific positions within arrays in MongoDB queries.
• Extract elements from arrays inside embedded (nested) documents.
• Create derived fields in aggregation results using specific array elements.
• Avoid processing entire arrays when only one value is needed.