$reverseArray (aggregation)

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Definition

Behavior
Example

Definition

$reverseArray

New in version 3.4.

Accepts an array expression as an argument and returns an array with the elements in reverse order.

$reverseArray has the following operator expression syntax:

{ $reverseArray: <array expression> }

The argument can be any valid expression as long as it resolves to an array.

Behavior

If the argument resolves to a value of null or refers to a missing field, $reverseArray returns null.

If the argument does not resolve to an array or null nor refers to a missing field, $reverseArray returns an error.

$reverseArray returns an empty array when the argument is an empty array.

If the argument contains subarrays, $reverseArray only operates on the top level array elements and will not reverse the contents of subarrays.

Example [1]

Results

{ $reverseArray: { $literal: [ 1, 2, 3 ] } }

[ 3, 2, 1 ]

{ $reverseArray:
   { $slice:  [ [ "foo", "bar", "baz", "qux" ], 1, 2 ] } }
}

[ "baz", "bar" ]

{ $reverseArray: null }

null

{ $reverseArray: { $literal: [ ] } }

[ ]

{ $reverseArray: { $literal: [ [ 1, 2, 3 ], [ 4, 5, 6 ] ] } }

[ [ 4, 5, 6 ], [ 1, 2, 3 ] ]

[1]	The examples in the table take a literal argument. To avoid parsing ambiguity if the literal argument is an array, you must wrap the literal array in a `$literal` expression or keep the outer array that designates the argument list (e.g. `[ [ 1, 2, 3 ] ]` ) to pass in the literal array `[1, 2, 3]`.

Example

A collection named users contains the following documents:

{ "_id" : 1, "name" : "dave123", "favorites" : [ "chocolate", "cake", "butter", "apples" ] }
{ "_id" : 2, "name" : "li", "favorites" : [ "apples", "pudding", "pie" ] }
{ "_id" : 3, "name" : "ahn", "favorites" : [ ] }
{ "_id" : 4, "name" : "ty" }

The following example returns an array containing the elements of the favorites array in reverse order:

db.users.aggregate([
   {
     $project:
      {
         name: 1,
         reverseFavorites: { $reverseArray: "$favorites" }
      }
   }
])

The operation returns the following results:

{ "_id" : 1, "name" : "dave123", "reverseFavorites" : [ "apples", "butter", "cake", "chocolate" ] }
{ "_id" : 2, "name" : "li", "reverseFavorites" : [ "pie", "pudding", "apples" ] }
{ "_id" : 3, "name" : "ahn", "reverseFavorites" : [ ] }
{ "_id" : 4, "name" : "ty", "reverseFavorites" : null }

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$round