Database Manual / Reference / Query Language / Expressions

$reverseArray (aggregation)

Definition

$reverseArray

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:

db.users.insertMany( [
   { _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 },
]