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$map

Definition

Evaluates an expression for each array element.

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

Syntax

{
  "$map": {
    "input": <input>,
    "as": <as>,
    "in": <in>
  }
}

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter Required Description
input Yes Expression supplying the value to process. A field reference such as "$price" reads from the current document.
as No Literal variable name for the current array item; defaults to "this". Refer to it as $$name in the nested expression.
in Yes Expression evaluated for each item using $$this or the variable named by as.

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 — $map 'input' must resolve to an array
missing Field absent Error — $map 'input' must resolve to an array
decimal 2.5 Error — $map 'input' must resolve to an array
numeric string "12" Error — $map 'input' must resolve to an array
text "hello" Error — $map 'input' must resolve to an array
boolean true Error — $map 'input' must resolve to an array
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $map 'input' must resolve to an array
empty array [] []
numeric array [1,2,3] [1,2,3]
object {"x":1} Error — $map 'input' must resolve to an array

Argument handling

Invocation Result
{"$map":[]} Error — $map expects an object with fields: input, as, in
{"$map":{"unexpected":1}} Error — $map requires 'input' and 'in' fields

Examples

Expression: 

{
  "$map": {
    "input": [
      1,
      2
    ],
    "as": "x",
    "in": "$$x"
  }
}

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: input

Input case Supplied value SrotaX result
null null Error — $map 'input' must resolve to an array
missing Field absent Error — $map 'input' must resolve to an array
zero 0 Error — $map 'input' must resolve to an array
negative -2 Error — $map 'input' must resolve to an array
decimal 2.5 Error — $map 'input' must resolve to an array
numeric string "12" Error — $map 'input' must resolve to an array
text "hello" Error — $map 'input' must resolve to an array
empty string "" Error — $map 'input' must resolve to an array
boolean true Error — $map 'input' must resolve to an array
false false Error — $map 'input' must resolve to an array
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $map 'input' must resolve to an array
decimal128 {"$numberDecimal":"1.25"} Error — $map 'input' must resolve to an array
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 {} Error — $map 'input' must resolve to an array
object {"x":1} Error — $map 'input' must resolve to an array
ISO date string "2026-01-15T15:00:00Z" Error — $map 'input' must resolve to an array
omitted named argument Argument removed Error — $map requires 'input' and 'in' fields

Argument: as

Input case Supplied value SrotaX result
null null Error — Cannot invoke "Object.toString()" because the return value of "java.util.Map.get(Object)" is null
zero 0 [null,null]
negative -2 [null,null]
decimal 2.5 [null,null]
numeric string "12" [null,null]
text "hello" [null,null]
empty string "" [null,null]
boolean true [null,null]
false false [null,null]
typed date {"$date":"2026-01-15T15:00:00Z"} [null,null]
decimal128 {"$numberDecimal":"1.25"} [null,null]
empty array [] [null,null]
numeric array [1,2,3] [null,null]
mixed array [1,null,"x",3] [null,null]
nested array [[1,2],[3]] [null,null]
empty object {} [null,null]
object {"x":1} [null,null]
ISO date string "2026-01-15T15:00:00Z" [null,null]
omitted named argument Argument removed [null,null]

Argument: in

Input case Supplied value SrotaX result
null null [null,null]
missing Field absent [null,null]
zero 0 [0,0]
negative -2 [-2,-2]
decimal 2.5 [2.5,2.5]
numeric string "12" ["12","12"]
text "hello" ["hello","hello"]
empty string "" ["",""]
boolean true [true,true]
false false [false,false]
typed date {"$date":"2026-01-15T15:00:00Z"} [{"$date":1768489200000},{"$date":1768489200000}]
decimal128 {"$numberDecimal":"1.25"} [{"$numberDecimal":"1.25"},{"$numberDecimal":"1.25"}]
empty array [] [[],[]]
numeric array [1,2,3] [[1,2,3],[1,2,3]]
mixed array [1,null,"x",3] [[1,null,"x",3],[1,null,"x",3]]
nested array [[1,2],[3]] [[[1,2],[3]],[[1,2],[3]]]
empty object {} [{},{}]
object {"x":1} [{"x":1},{"x":1}]
ISO date string "2026-01-15T15:00:00Z" ["2026-01-15T15:00:00Z","2026-01-15T15:00:00Z"]
omitted named argument Argument removed Error — $map requires 'input' and 'in' fields

Additional boundary examples

Show 1 tested examples

Expression: 

{
  "$map": {
    "input": [
      {
        "price": 2
      }
    ],
    "as": "item",
    "in": {
      "$multiply": [
        "$$item.price",
        3
      ]
    }
  }
}

Input: 

{}

SrotaX: [6.0].

Verification

This page records 63 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: 24 exact BSON matches, 18 both rejected, 21 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 / null Error — $map 'input' must resolve to an array null
input / missing Error — $map 'input' must resolve to an array null
as / zero [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / negative [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / decimal [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / numeric string [null,null] Error — Invalid $project :: caused by :: '12' starts with an invalid character for a user variable name
as / text [null,null] Error — Invalid $project :: caused by :: Use of undefined variable: x
as / empty string [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / boolean [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / false [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / typed date [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / decimal128 [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / empty array [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / numeric array [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / mixed array [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / nested array [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / empty object [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / object [null,null] Error — Invalid $project :: caused by :: empty variable names are not allowed
as / ISO date string [null,null] Error — Invalid $project :: caused by :: '2026-01-15T15:00:00Z' starts with an invalid character for a user variable name
as / omitted named argument [null,null] Error — Invalid $project :: caused by :: Use of undefined variable: x
all / boundary example [6.0] [6]

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