Skip to content

$reduce

The $reduce operator applies an expression to each element in an array and accumulates the result.

📌 Syntax

{
  "$reduce": {
    "input": <arrayExpression>,
    "initialValue": <expression>,
    "in": <accumulatorExpression>
  }
}
  • input: The array to iterate
  • initialValue: The starting value of the accumulator
  • in: Expression that updates the accumulator using $$value and $$this

✅ Base Example – Sum Values

📥 Input Document

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

📌 Expression

{
  "$reduce": {
    "input": "$nums",
    "initialValue": 0,
    "in": { "$add": ["$$value", "$$this"] }
  }
}

📤 Output

10

✅ Base Example – Concatenate Strings

📥 Input Document

{ "words": ["hello", "world"] }

📌 Expression

{
  "$reduce": {
    "input": "$words",
    "initialValue": "",
    "in": { "$concat": ["$$value", " ", "$$this"] }
  }
}

📤 Output

" hello world"

🧱 Ecommerce Example – Total Price of All Items

📌 Pipeline

[
  {
    "$project": {
      "total": {
        "$reduce": {
          "input": "$items",
          "initialValue": 0,
          "in": {
            "$add": ["$$value", { "$multiply": ["$$this.price", "$$this.quantity"] }]
          }
        }
      }
    }
  }
]

📥 Input Document

{
  "items": [
    { "name": "Book", "price": 20, "quantity": 2 },
    { "name": "Bag", "price": 50, "quantity": 1 }
  ]
}

📤 Output

{ "total": 90 }

🔧 Common Use Cases

  • Compute totals or nested accumulations
  • String aggregation
  • Custom transformations
  • $map, $filter, $concat, $sum, $avg

🧠 Notes

  • $$value refers to the accumulator
  • $$this refers to the current array element