$arrayElemAt

Definition

Reads an array item at a positive or negative index.

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

Syntax

{ "$arrayElemAt": [ <array>, <index> ] }

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter	Required	Description
array	Yes	Expression supplying the source array.
index	Yes	Zero-based array position. A negative position counts from the end.

Arguments are positional: keep the order shown. The authoring signature accepts 2–2 arguments.

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 array argument:

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

Argument handling

Invocation	Result
{"$arrayElemAt":[]}	Error — $arrayElemAt expects [array, index]
{"$arrayElemAt":{"unexpected":1}}	Error — $arrayElemAt expects [array, index]

Examples

Expression:

{
  "$arrayElemAt": [
    [
      1,
      2
    ],
    -1
  ]
}

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: array[0]

Input case	Supplied value	SrotaX result
null	null	null
missing	Field absent	null
zero	0	Error — $arrayElemAt requires a List and an integer index
negative	-2	Error — $arrayElemAt requires a List and an integer index
decimal	2.5	Error — $arrayElemAt requires a List and an integer index
numeric string	"12"	Error — $arrayElemAt requires a List and an integer index
text	"hello"	Error — $arrayElemAt requires a List and an integer index
empty string	""	Error — $arrayElemAt requires a List and an integer index
boolean	true	Error — $arrayElemAt requires a List and an integer index
false	false	Error — $arrayElemAt requires a List and an integer index
typed date	{"$date":"2026-01-15T15:00:00Z"}	Error — $arrayElemAt requires a List and an integer index
decimal128	{"$numberDecimal":"1.25"}	Error — $arrayElemAt requires a List and an integer index
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	{}	Error — $arrayElemAt requires a List and an integer index
object	{"x":1}	Error — $arrayElemAt requires a List and an integer index
ISO date string	"2026-01-15T15:00:00Z"	Error — $arrayElemAt requires a List and an integer index

Argument: index[1]

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

Additional boundary examples

Show 1 tested examples

Expression:

{
  "$arrayElemAt": [
    [
      1
    ],
    5
  ]
}

Input:

{}

SrotaX: null.

Verification

This page records 42 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: 8 exact BSON matches, 30 both rejected, 4 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
array[0] / empty array	null	Missing (field omitted)
index[1] / null	Error — $arrayElemAt requires a List and an integer index	null
index[1] / missing	Error — $arrayElemAt requires a List and an integer index	null
all / boundary example	null	Missing (field omitted)

Download full comparison evidence.