$concatArrays

Definition

Concatenates arrays.

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

Syntax

{ "$concatArrays": [ <array>, ... ] }

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter	Required	Description
array	See argument count below	Expression supplying the source array.

Arguments are positional: keep the order shown. Supply 0 or more 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 — $concatArrays only works with arrays
numeric string	"12"	Error — $concatArrays only works with arrays
text	"hello"	Error — $concatArrays only works with arrays
boolean	true	Error — $concatArrays only works with arrays
typed date	{"$date":"2026-01-15T15:00:00Z"}	Error — $concatArrays only works with arrays
empty array	[]	[2]
numeric array	[1,2,3]	[1,2,3,2]
object	{"x":1}	Error — $concatArrays only works with arrays
null	null	null
missing	Field absent	null
decimal	2.5	Error — $concatArrays only works with arrays
numeric string	"12"	Error — $concatArrays only works with arrays
text	"hello"	Error — $concatArrays only works with arrays
boolean	true	Error — $concatArrays only works with arrays
typed date	{"$date":"2026-01-15T15:00:00Z"}	Error — $concatArrays only works with arrays
empty array	[]	[1]
numeric array	[1,2,3]	[1,1,2,3]
object	{"x":1}	Error — $concatArrays only works with arrays

Argument handling

Invocation	Result
{"$concatArrays":[]}	[]
{"$concatArrays":{"unexpected":1}}	Error — $concatArrays expects a list of arrays

Examples

Expression:

{
  "$concatArrays": [
    [
      1
    ],
    [
      2
    ]
  ]
}

Input document:

{}

Observed SrotaX result: [1,2].

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 — $concatArrays only works with arrays
negative	-2	Error — $concatArrays only works with arrays
decimal	2.5	Error — $concatArrays only works with arrays
numeric string	"12"	Error — $concatArrays only works with arrays
text	"hello"	Error — $concatArrays only works with arrays
empty string	""	Error — $concatArrays only works with arrays
boolean	true	Error — $concatArrays only works with arrays
false	false	Error — $concatArrays only works with arrays
typed date	{"$date":"2026-01-15T15:00:00Z"}	Error — $concatArrays only works with arrays
decimal128	{"$numberDecimal":"1.25"}	Error — $concatArrays only works with arrays
empty array	[]	[2]
numeric array	[1,2,3]	[1,2,3,2]
mixed array	[1,null,"x",3]	[1,null,"x",3,2]
nested array	[[1,2],[3]]	[[1,2],[3],2]
empty object	{}	Error — $concatArrays only works with arrays
object	{"x":1}	Error — $concatArrays only works with arrays
ISO date string	"2026-01-15T15:00:00Z"	Error — $concatArrays only works with arrays

Argument: array[1]

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

Verification

This page records 41 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: 14 exact BSON matches, 27 both rejected, 0 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.

No differences were observed in the compared cases.

Download full comparison evidence.