$expr

Definition

Evaluates an expression as a Fluxion passthrough extension.

Expression operator · Advanced · Studio Visual: not exposed. Studio validates the complete expression separately.

Syntax

{ "$expr": <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	2.5 (double)
numeric string	"12"	"12"
text	"hello"	"hello"
boolean	true	true
typed date	{"$date":"2026-01-15T15:00:00Z"}	{'$numberLong': '1768489200000'} (date)
empty array	[]	[]
numeric array	[1,2,3]	[1,2,3]
object	{"x":1}	{"x":1}

Argument handling

Invocation	Result
{"$expr":[]}	[]
{"$expr":{"unexpected":1}}	{"unexpected":1}

Examples

Expression:

{
  "$expr": {
    "$add": [
      1,
      2
    ]
  }
}

Input document:

{}

Observed SrotaX result: 3.0 (double).

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	0 (int32)
negative	-2	-2 (int32)
decimal	2.5	2.5 (double)
numeric string	"12"	"12"
text	"hello"	"hello"
empty string	""	""
boolean	true	true
false	false	false
typed date	{"$date":"2026-01-15T15:00:00Z"}	{'$numberLong': '1768489200000'} (date)
decimal128	{"$numberDecimal":"1.25"}	1.25 (decimal128)
empty array	[]	[]
numeric array	[1,2,3]	[1,2,3]
mixed array	[1,null,"x",3]	[1,null,"x",3]
nested array	[[1,2],[3]]	[[1,2],[3]]
empty object	{}	{}
object	{"x":1}	{"x":1}
ISO date string	"2026-01-15T15:00:00Z"	"2026-01-15T15:00:00Z"

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

This name is a SrotaX extension or belongs to another MongoDB context. It is not compared as a MongoDB aggregation expression. This is not a missing implementation failure.