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$mod

The $mod operator returns the remainder of the division of one number by another.

📌 Syntax

{ "$mod": [ <dividend>, <divisor> ] }

Both values must resolve to numbers. Division by zero returns an error.

✅ Base Example 1 – Modulo with Even/Odd Check

📥 Input Document

{ "orderId": 101 }

📌 Expression

{ "$mod": ["$orderId", 2] }

📤 Output

1

Output 0 means even, 1 means odd.

✅ Base Example 2 – Minutes Past the Hour

📥 Input Document

{ "elapsedSeconds": 3670 }

📌 Expression

{ "$mod": ["$elapsedSeconds", 3600] }

📤 Output

70

🧱 Ecommerce Example – Discount Every 3rd Item

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$project": {
      "name": "$items.name",
      "isEligibleForDiscount": {
        "$eq": [{ "$mod": ["$items.index", 3] }, 0]
      }
    }
  }
]

📥 Input Document

{
  "items": [
    { "name": "Item A", "index": 0 },
    { "name": "Item B", "index": 1 },
    { "name": "Item C", "index": 2 },
    { "name": "Item D", "index": 3 }
  ]
}

📤 Output

[
  { "name": "Item A", "isEligibleForDiscount": true },
  { "name": "Item B", "isEligibleForDiscount": false },
  { "name": "Item C", "isEligibleForDiscount": false },
  { "name": "Item D", "isEligibleForDiscount": true }
]

🔧 Common Use Cases

  • Even/odd classification
  • Recurring logic (every N items)
  • Time breakdowns (minutes/seconds)
  • $divide, $add, $subtract, $cond
  • $eq, $project, $switch

🧠 Notes

  • Dividing by zero will return an error.
  • Common in coupon, rule, and scheduling systems.