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$concat

The $concat operator joins strings together into a single string.

📌 Syntax

{ "$concat": [ <expression1>, <expression2>, ... ] }

All expressions must resolve to strings. If any operand resolves to null, the result is null.

✅ Base Example 1 – Full Name

📥 Input Document

{ "firstName": "Jane", "lastName": "Doe" }

📌 Expression

{ "$concat": ["$firstName", " ", "$lastName"] }

📤 Output

"Jane Doe"

✅ Base Example 2 – File Path Builder

📥 Input Document

{ "folder": "invoices", "file": "2025.pdf" }

📌 Expression

{ "$concat": ["/data/", "$folder", "/", "$file"] }

📤 Output

"/data/invoices/2025.pdf"

🧱 Ecommerce Example – Generate SKU Code

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$project": {
      "sku": {
        "$concat": [
          "$items.category", "-",
          "$items.brand", "-",
          "$items.productId"
        ]
      }
    }
  }
]

📥 Input Document

{
  "items": [
    {
      "productId": "X123",
      "category": "ELEC",
      "brand": "SNY"
    },
    {
      "productId": "Y987",
      "category": "BOOK",
      "brand": "PNH"
    }
  ]
}

📤 Output

[
  { "sku": "ELEC-SNY-X123" },
  { "sku": "BOOK-PNH-Y987" }
]

🔧 Common Use Cases

  • Formatting names and addresses
  • Constructing SKUs or file paths
  • HTML or CSV generation
  • $toString, $substr, $toLower, $trim
  • $project, $set, $map

🧠 Notes

  • Ensure all values are strings or cast them using $toString.
  • Use $cond or $ifNull to avoid null operands.