$concatArrays

The $concatArrays operator merges two or more arrays into a single array.

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📌 Syntax

{ "$concatArrays": [ <array1>, <array2>, ... ] }

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✅ Base Example – Merge Lists

📥 Input Document

{ "a": [1, 2], "b": [3, 4] }

📌 Expression

{ "$concatArrays": ["$a", "$b"] }

📤 Output

[1, 2, 3, 4]

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✅ Base Example – Add Single Item

📥 Input Document

{ "items": ["Pen", "Book"] }

📌 Expression

{ "$concatArrays": ["$items", ["Notebook"]] }

📤 Output

["Pen", "Book", "Notebook"]

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🧱 Ecommerce Example – Combine Inventory Locations

📌 Pipeline

[
  {
    "$project": {
      "product": "$name",
      "allLocations": {
        "$concatArrays": ["$warehouseA", "$warehouseB"]
      }
    }
  }
]

📥 Input Document

{
  "name": "Speaker",
  "warehouseA": ["W1", "W2"],
  "warehouseB": ["W3"]
}

📤 Output

{
  "product": "Speaker",
  "allLocations": ["W1", "W2", "W3"]
}

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🔧 Common Use Cases

• Merge multiple inventories
• Combine feature lists
• Append new values to arrays

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🔗 Related Operators

• $arrayElemAt, $slice, $map, $reduce, $literal

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🧠 Notes

• Works only on arrays; input expressions must evaluate to arrays.
• Null or missing inputs are treated as null and return null.

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