Skip to content

$last

Definition

Reads the last array element.

Expression operator · Arrays · Studio Visual: available. Studio validates the complete expression separately.

Syntax

{ "$last": <expression> }

Replace angle-bracket placeholders with expressions or values.

Expression supplying the value to process. A field reference such as "$price" reads from the current document.

The value goes directly after the operator name; do not add an input wrapper.

Behavior

The following observations use the example expression below. Each row substitutes one argument while retaining the other example arguments. A missing field differs from an explicit null; the result shown is what Core actually returned.

For the input argument:

Input case Supplied value Result
null null null
missing Field absent null
decimal 2.5 null
numeric string "12" null
text "hello" null
boolean true null
typed date {"$date":"2026-01-15T15:00:00Z"} null
empty array [] null
numeric array [1,2,3] 3 (int32)
object {"x":1} null

Argument handling

Invocation Result
{"$last":[]} null
{"$last":{"unexpected":1}} null

Examples

Expression: 

{
  "$last": {
    "$literal": [
      1,
      2
    ]
  }
}

Input document: 

{}

Observed SrotaX result: 2 (int32).

Detailed input cases

Expand the complete argument-by-argument test matrix

Each table changes only the indicated argument from the example above. Other arguments retain their baseline values. Values normally come from the input field probe; literal configuration arguments are replaced directly. “Missing” means the field is absent, not explicitly null. Numeric strings are strings, not numbers.

Argument: input

Input case Supplied value SrotaX result
null null null
missing Field absent null
zero 0 null
negative -2 null
decimal 2.5 null
numeric string "12" null
text "hello" null
empty string "" null
boolean true null
false false null
typed date {"$date":"2026-01-15T15:00:00Z"} null
decimal128 {"$numberDecimal":"1.25"} null
empty array [] null
numeric array [1,2,3] 3 (int32)
mixed array [1,null,"x",3] 3 (int32)
nested array [[1,2],[3]] [3]
empty object {} null
object {"x":1} null
ISO date string "2026-01-15T15:00:00Z" null

Verification

This page records 22 executed cases. It covers individual argument substitutions, not every cross-product of arguments, locale, daylight-saving transitions, numeric overflow, collation or resource limits. No untested behavior is guaranteed.

Download exact expressions, inputs and SrotaX results. Errors are observed messages, not stable public error codes. The reference generator reads the checked-in execution report; do not edit generated observations by hand.

MongoDB differences

Compared with MongoDB 9.0.4: 6 exact BSON matches, 0 both rejected, 16 value/type/acceptance differences, 0 object-field-order differences.

“Exact” includes numeric BSON types and object key order. A value of 3 as int32 differs from 3.0 as double. Object-field-order differences are reported separately; object key ordering is not enforced by the engine regression baseline. Both rejected does not mean the error codes or messages match. This is finite-case evidence, not full compatibility certification.

Show observed differences
Tested case SrotaX MongoDB
input / zero null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is int
input / negative null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is int
input / decimal null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is double
input / numeric string null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is string
input / text null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is string
input / empty string null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is string
input / boolean null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is bool
input / false null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is bool
input / typed date null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is date
input / decimal128 null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is decimal
input / empty array null Missing (field omitted)
input / empty object null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is object
input / object null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is object
input / ISO date string null Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $last's argument must be an array, but is string
all / empty arguments null Error — Invalid $project :: caused by :: Expression $last takes exactly 1 arguments. 0 were passed in.
all / unknown option null Error — Failed to optimize pipeline :: caused by :: $last's argument must be an array, but is object

Download full comparison evidence.