$accumulator (Custom Accumulator)

Last updated: 2026-10-09

$accumulator (Custom Accumulator)

$accumulator defines custom accumulator logic with JavaScript callbacks — init, accumulate, merge, finalize — inside $group. It covers aggregation logic that built-in accumulators cannot express. It runs server-side JavaScript and is slower than native accumulators.

📝 Syntax

{ $accumulator: { init: <function>, accumulate: <function>, accumulateArgs: [ <expression> ], merge: <function>, finalize: <function>, lang: "js" } }

Returns：Custom accumulation result (type decided by your functions)

▶ Example

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

// $accumulator: count distinct prices per item with a Set
db.sales.aggregate([
  { $group: { _id: '$item', uniq: {
    $accumulator: {
      init: function() { return new Set(); },
      accumulate: function(set, v) { set.add(v); },
      accumulateArgs: ['$price'],
      merge: function(a, b) { for (var v of b) a.add(v); },
      finalize: function(set) { return set.size; },
      lang: 'js'
    }
  } }
])

Output：

// Count of distinct prices per item

📌 Notes

• Runs on server-side JavaScript, slower than native accumulators
• Callbacks must be serializable and cannot capture outer variables
• The merge callback combines group results in sharded or distributed setups

🏷️ Related Tags

💡 Related Cases

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