Accumulators

Accumulators calculate aggregated values like sums, maxima, and minima across a variable number of input values. You can use accumulators in these aggregation pipeline stages:

Name	Description
`$accumulator`	Returns the result of a user-defined accumulator function.
`$addToSet`	Returns an array of unique expression values for each group. Order of the array elements is undefined.
`$avg`	Returns an average of numerical values.
`$bottom`	Returns the bottom element within a group according to the specified sort order.
`$bottomN`	Returns an aggregation of the bottom `n` fields within a group, according to the specified sort order.
`$concatArrays`	Returns a single array that combines the elements of two or more arrays. New in version 8.1.
`$count`	Returns the number of documents in a group. Distinct from the `$count` pipeline stage.
`$first`	Returns the result of an expression for the first document in a group.
`$firstN`	Returns an aggregation of the first `n` elements within a group. Only meaningful when documents are in a defined order. Distinct from the `$firstN` array operator.
`$last`	Returns the result of an expression for the last document in a group.
`$lastN`	Returns the result of evaluating a specified expression over the last n elements in a group.
`$max`	Returns the highest expression value for each group.
`$maxN`	Returns an aggregation of the `n` maximum valued elements in a group. Distinct from the `$maxN` array operator.
`$median`	Returns an approximation of the median (50th percentile) value of documents in a group. New in version 7.0.
`$mergeObjects`	Returns a document created by combining the input documents for each group.
`$min`	Returns the lowest expression value for each group.
`$minN`	Returns an aggregation of the `n` minimum valued elements in a group. Distinct from the `$minN` array operator.
`$percentile`	Returns an array of scalar values that correspond to specified percentile values. New in version 7.0.
`$push`	Returns an array of expression values for documents in each group.
`$setUnion`	Takes two or more arrays and returns an array containing the elements that appear in any input array. New in version 8.1.
`$stdDevPop`	Returns the population standard deviation of values from documents in a group.
`$stdDevSamp`	Returns the sample standard deviation of values from documents in a group.
`$sum`	Returns a sum of numerical values. Ignores non-numeric values.
`$top`	Returns the top element within a group according to the specified sort order.
`$topN`	Returns an aggregation of the top `n` fields within a group, according to the specified sort order.

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$accumulator