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$size

The $size operator returns the number of elements in an array.

📌 Syntax

{ "$size": <arrayExpression> }

The input must resolve to an array.

✅ Base Example – Count Elements

📥 Input Document

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

📌 Expression

{ "$size": "$tags" }

📤 Output

3

✅ Base Example – Empty Array

📥 Input Document

{ "items": [] }

📌 Expression

{ "$size": "$items" }

📤 Output

0

🧱 Ecommerce Example – Count Item Features

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$project": {
      "product": "$items.name",
      "featureCount": { "$size": "$items.features" }
    }
  }
]

📥 Input Document

{
  "items": [
    {
      "name": "Backpack",
      "features": ["Padded", "Waterproof", "Adjustable Straps"]
    },
    {
      "name": "Watch",
      "features": []
    }
  ]
}

📤 Output

[
  { "product": "Backpack", "featureCount": 3 },
  { "product": "Watch", "featureCount": 0 }
]

🔧 Common Use Cases

  • Count number of tags, features, options
  • Validate minimum/maximum items
  • Use with $cond or $switch for conditional logic
  • $isArray, $map, $filter, $arrayElemAt

🧠 Notes

  • Returns an error if input is not an array
  • Use $ifNull or $cond for safety