How to Use $arrayElemAt in Aggregation in MongoDB

In MongoDB aggregation, use $arrayElemAt to get the element at a specific index from an array. It takes two arguments: the array and the zero-based index. For example, { $arrayElemAt: ["$arrayField", 1] } returns the second element of arrayField.

Syntax

The $arrayElemAt operator takes two arguments inside an array:

• array: The array field or expression you want to access.
• index: The zero-based position of the element you want to retrieve.

It returns the element at the specified index or null if the index is out of bounds.

json

{ $arrayElemAt: [ <array>, <index> ] }

Example

This example shows how to use $arrayElemAt in an aggregation pipeline to get the second element from an array field called scores in each document.

mongodb

db.students.aggregate([
  {
    $project: {
      name: 1,
      secondScore: { $arrayElemAt: ["$scores", 1] }
    }
  }
])

Output

[
  { "_id": ObjectId("..."), "name": "Alice", "secondScore": 85 },
  { "_id": ObjectId("..."), "name": "Bob", "secondScore": 90 },
  { "_id": ObjectId("..."), "name": "Charlie", "secondScore": null }
]

Common Pitfalls

• Using a negative index without understanding it returns elements from the end (e.g., -1 returns the last element).
• Index out of bounds returns null, which might cause unexpected results if not handled.
• Passing a non-array value as the first argument causes errors.

Always ensure the field is an array and the index is within range.

mongodb

/* Wrong: index out of bounds returns null */
db.collection.aggregate([
  { $project: { elem: { $arrayElemAt: ["$arrayField", 10] } } }
])

/* Right: check array length before accessing or handle null */

Quick Reference

Usage	Description
{ $arrayElemAt: [ , ] }	Returns element at zero-based from
Negative index	Counts from the end (-1 is last element)
Index out of range	Returns null
Non-array input	Causes error

Key Takeaways

Use $arrayElemAt to get an element at a specific zero-based index from an array in aggregation.

Negative indexes count from the end of the array (-1 is last element).

If the index is out of bounds, $arrayElemAt returns null.

Ensure the first argument is an array to avoid errors.

Use $arrayElemAt inside $project or other pipeline stages to extract array elements.