$reduce (Reduce Array to Single Value)

Last updated: 2026-10-09

$reduce (Reduce Array to Single Value)

$reduce folds an array into a single value, the aggregation equivalent of functional reduce/fold. It takes an initialValue and an in expression where $$value is the accumulator and $$this is the current element. Typical uses: summing an array of numbers, joining strings, or counting elements that satisfy a condition. An empty array returns initialValue directly. $reduce is more general than $sum or $concat and can express arbitrary accumulation logic.

📝 Syntax

{ $reduce: { input: <array>, initialValue: <initialValue>, in: <reducer> } }

⚙️ Parameters

Returns：A single reduced value; empty array returns initialValue; null input yields null

▶ Example

Run the commands below in your local mongosh (requires a running mongod):

// $reduce: sum the array
db.stats.aggregate([
  { $project: {
      total: { $reduce: { input: '$scores', initialValue: 0, in: { $add: ['$$value', '$$this'] } } }
  } }
])

Output：

{ '_id': 1, 'total': 285 }

📌 Notes

• Inside in, $$value is the accumulator and $$this is the current element; both are built-in
• An empty array returns initialValue without error
• The type of initialValue drives the result type: 0 yields numbers, '' yields strings

🏷️ Related Tags

💡 Related Cases

📚 Related Tutorials

❓ FAQ

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