$size

Definition

Counts elements of an array.

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

Syntax

{ "$size": <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	Error — $size requires an array
missing	Field absent	Error — $size requires an array
decimal	2.5	Error — $size requires an array
numeric string	"12"	Error — $size requires an array
text	"hello"	Error — $size requires an array
boolean	true	Error — $size requires an array
typed date	{"$date":"2026-01-15T15:00:00Z"}	Error — $size requires an array
empty array	[]	0 (int32)
numeric array	[1,2,3]	3 (int32)
object	{"x":1}	Error — $size requires an array

Argument handling

Invocation	Result
{"$size":[]}	Error — $size requires one argument
{"$size":{"unexpected":1}}	Error — $size requires an array

Examples

Expression:

{
  "$size": {
    "$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	Error — $size requires an array
missing	Field absent	Error — $size requires an array
zero	0	Error — $size requires an array
negative	-2	Error — $size requires an array
decimal	2.5	Error — $size requires an array
numeric string	"12"	Error — $size requires an array
text	"hello"	Error — $size requires an array
empty string	""	Error — $size requires an array
boolean	true	Error — $size requires an array
false	false	Error — $size requires an array
typed date	{"$date":"2026-01-15T15:00:00Z"}	Error — $size requires an array
decimal128	{"$numberDecimal":"1.25"}	Error — $size requires an array
empty array	[]	0 (int32)
numeric array	[1,2,3]	3 (int32)
mixed array	[1,null,"x",3]	4 (int32)
nested array	[[1,2],[3]]	2 (int32)
empty object	{}	Error — $size requires an array
object	{"x":1}	Error — $size requires an array
ISO date string	"2026-01-15T15:00:00Z"	Error — $size requires an array

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: 5 exact BSON matches, 17 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.