$reduce

Same idea as reduce in JavaScript: walk the array, carry a running value. In MongoDB that running value is $$value and the current element is $$this, and the syntax is verbose enough that it pays to read it slowly once.

In JavaScript you would write:

array.reduce(...)

Suppose:

{
  scores: [10, 20, 30]
}

Calculate:

10 + 20 + 30

{
  $project: {
    total: {
      $reduce: {
        input: "$scores",
        initialValue: 0,

        in: {
          $add: [
            "$$value",
            "$$this"
          ]
        }
      }
    }
  }
}

Conceptually:

scores.reduce(
  (value, current) => value + current,
  0
)

MongoDB variables:

$$value
→ accumulator

$$this
→ current array element

Example: Order Total Without $unwind

Given:

{
  items: [
    {
      price: 1000,
      quantity: 1
    },
    {
      price: 50,
      quantity: 2
    }
  ]
}

Calculate order total:

{
  $set: {
    orderTotal: {
      $reduce: {
        input: "$items",
        initialValue: 0,

        in: {
          $add: [
            "$$value",

            {
              $multiply: [
                "$$this.price",
                "$$this.quantity"
              ]
            }
          ]
        }
      }
    }
  }
}

Result:

1000 + (50 × 2)

= 1100

When $reduce Is Actually Useful

Don’t force $reduce everywhere.

Use it when:

You have an array inside one document and need to calculate one value from that array.

Examples:

Calculate order total
Calculate total duration
Calculate combined score
Concatenate array values
Build a custom object

If you need to aggregate across multiple documents, think:

$group

If you need to reduce one array, think:

$reduce

Very important distinction:

Across documents
→ $group

Inside one array
→ $reduce

Try it

db.orders.aggregate([
  {
    $project: {
      itemCount: {
        $reduce: {
          input: "$items",
          initialValue: 0,
          in: { $add: ["$$value", "$$this.quantity"] }
        }
      }
    }
  },
  { $limit: 5 }
])

$$value is the running total, $$this is the element. Change $add to $max and the same three lines give you the largest quantity in the order instead of the sum.