$arrayElemAt

The $arrayElemAt operator returns the element at a specified index in an array.

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📌 Syntax

{ "$arrayElemAt": [ <arrayExpression>, <index> ] }

• Indexing starts at 0.
• Negative indexes count from the end.

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✅ Base Example – Get First Tag

📥 Input Document

{ "tags": ["sale", "new", "trending"] }

📌 Expression

{ "$arrayElemAt": ["$tags", 0] }

📤 Output

"sale"

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✅ Base Example – Get Last Element

📥 Input Document

{ "tags": ["sale", "new", "trending"] }

📌 Expression

{ "$arrayElemAt": ["$tags", -1] }

📤 Output

"trending"

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🧱 Ecommerce Example – Access Top-Scoring Review

📌 Pipeline

[
  {
    "$project": {
      "product": "$name",
      "topReview": { "$arrayElemAt": ["$reviews", 0] }
    }
  }
]

📥 Input Document

{
  "name": "Laptop",
  "reviews": [
    { "user": "Alice", "rating": 5 },
    { "user": "Bob", "rating": 4 }
  ]
}

📤 Output

{
  "product": "Laptop",
  "topReview": { "user": "Alice", "rating": 5 }
}

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🔧 Common Use Cases

• Access top/last items
• Index-based lookups
• Extract highlights or featured content

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🔗 Related Operators

• $slice, $filter, $map, $reduce

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🧠 Notes

• Index out of bounds returns null.
• Combine with $size or $cond for safer access.

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