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$map

The $map operator applies a transformation expression to each element of an input array.

📌 Syntax

{
  "$map": {
    "input": <arrayExpression>,
    "as": <variableName>,
    "in": <expression>
  }
}
  • input: The array to iterate
  • as: The name of the variable for each element
  • in: The expression to evaluate for each element

✅ Base Example – Multiply All by 2

📥 Input Document

{ "values": [1, 2, 3, 4] }

📌 Expression

{
  "$map": {
    "input": "$values",
    "as": "num",
    "in": { "$multiply": ["$$num", 2] }
  }
}

📤 Output

[2, 4, 6, 8]

✅ Base Example – Uppercase Items

📥 Input Document

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

📌 Expression

{
  "$map": {
    "input": "$tags",
    "as": "tag",
    "in": { "$toUpper": "$$tag" }
  }
}

📤 Output

["SALE", "NEW", "HOT"]

🧱 Ecommerce Example – Compute Total per Item

📌 Pipeline

[
  {
    "$project": {
      "orderId": 1,
      "items": {
        "$map": {
          "input": "$items",
          "as": "item",
          "in": {
            "name": "$$item.name",
            "total": {
              "$multiply": ["$$item.price", "$$item.quantity"]
            }
          }
        }
      }
    }
  }
]

📥 Input Document

{
  "orderId": 123,
  "items": [
    { "name": "Pen", "price": 2, "quantity": 10 },
    { "name": "Notebook", "price": 5, "quantity": 3 }
  ]
}

📤 Output

{
  "orderId": 123,
  "items": [
    { "name": "Pen", "total": 20 },
    { "name": "Notebook", "total": 15 }
  ]
}

🔧 Common Use Cases

  • Transform items in an array
  • Compute fields per subdocument
  • Prepare output for $project or $group
  • $filter, $reduce, $concatArrays, $arrayElemAt

🧠 Notes

  • Use $$ prefix when referencing variables in $map.
  • Input must be an array; otherwise, returns null.