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$function

Definition

Executes a JavaScript function in a restricted GraalJS context.

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

Syntax

{
  "$function": {
    "body": <body>,
    "args": <args>,
    "lang": <lang>
  }
}

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter Required Description
body Yes Literal function body understood by the engine function implementation.
args Yes Array of expressions passed as function arguments.
lang Yes Literal function-language selector.

Supported values

lang must be "js" (case-insensitive). body is a JavaScript function string, for example "function(x) { return x + 1; }". Arguments are evaluated before invocation. The engine applies a two-second execution timeout.

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 body argument:

Input case Supplied value Result
null {"body":null,"args":[1],"lang":"js"} Error — $function 'body' must be a JavaScript function body
decimal {"body":2.5,"args":[1],"lang":"js"} Error — $function 'body' must be a JavaScript function body
numeric string {"body":"12","args":[1],"lang":"js"} Error — JavaScript function execution failed
text {"body":"hello","args":[1],"lang":"js"} Error — JavaScript function execution failed
boolean {"body":true,"args":[1],"lang":"js"} Error — $function 'body' must be a JavaScript function body
typed date {"body":{"$date":"2026-01-15T15:00:00Z"},"args":[1],"lang":"js"} Error — $function 'body' must be a JavaScript function body
empty array {"body":[],"args":[1],"lang":"js"} Error — $function 'body' must be a JavaScript function body
numeric array {"body":[1,2,3],"args":[1],"lang":"js"} Error — $function 'body' must be a JavaScript function body
object {"body":{"x":1},"args":[1],"lang":"js"} Error — $function 'body' must be a JavaScript function body

Argument handling

Invocation Result
{"$function":[]} Error — $function expects a document with 'body', 'args', and 'lang'
{"$function":{"unexpected":1}} Error — $function 'lang' must be specified and set to 'js'

Examples

Expression: 

{
  "$function": {
    "body": "function(a) { return a + 1; }",
    "args": [
      1
    ],
    "lang": "js"
  }
}

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: body

Input case Supplied value SrotaX result
null null Error — $function 'body' must be a JavaScript function body
zero 0 Error — $function 'body' must be a JavaScript function body
negative -2 Error — $function 'body' must be a JavaScript function body
decimal 2.5 Error — $function 'body' must be a JavaScript function body
numeric string "12" Error — JavaScript function execution failed
text "hello" Error — JavaScript function execution failed
empty string "" Error — JavaScript function execution failed
boolean true Error — $function 'body' must be a JavaScript function body
false false Error — $function 'body' must be a JavaScript function body
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $function 'body' must be a JavaScript function body
decimal128 {"$numberDecimal":"1.25"} Error — $function 'body' must be a JavaScript function body
empty array [] Error — $function 'body' must be a JavaScript function body
numeric array [1,2,3] Error — $function 'body' must be a JavaScript function body
mixed array [1,null,"x",3] Error — $function 'body' must be a JavaScript function body
nested array [[1,2],[3]] Error — $function 'body' must be a JavaScript function body
empty object {} Error — $function 'body' must be a JavaScript function body
object {"x":1} Error — $function 'body' must be a JavaScript function body
ISO date string "2026-01-15T15:00:00Z" Error — JavaScript function execution failed
omitted named argument Argument removed Error — $function 'body' must be a JavaScript function body

Argument: args

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

Argument: lang

Input case Supplied value SrotaX result
null null Error — $function 'lang' must be specified and set to 'js'
zero 0 Error — $function 'lang' must be specified and set to 'js'
negative -2 Error — $function 'lang' must be specified and set to 'js'
decimal 2.5 Error — $function 'lang' must be specified and set to 'js'
numeric string "12" Error — $function 'lang' must be specified and set to 'js'
text "hello" Error — $function 'lang' must be specified and set to 'js'
empty string "" Error — $function 'lang' must be specified and set to 'js'
boolean true Error — $function 'lang' must be specified and set to 'js'
false false Error — $function 'lang' must be specified and set to 'js'
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $function 'lang' must be specified and set to 'js'
decimal128 {"$numberDecimal":"1.25"} Error — $function 'lang' must be specified and set to 'js'
empty array [] Error — $function 'lang' must be specified and set to 'js'
numeric array [1,2,3] Error — $function 'lang' must be specified and set to 'js'
mixed array [1,null,"x",3] Error — $function 'lang' must be specified and set to 'js'
nested array [[1,2],[3]] Error — $function 'lang' must be specified and set to 'js'
empty object {} Error — $function 'lang' must be specified and set to 'js'
object {"x":1} Error — $function 'lang' must be specified and set to 'js'
ISO date string "2026-01-15T15:00:00Z" Error — $function 'lang' must be specified and set to 'js'
omitted named argument Argument removed Error — $function 'lang' must be specified and set to 'js'

Verification

This page records 61 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: 0 exact BSON matches, 55 both rejected, 6 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
all / baseline 2 (int32) 2.0 (double)
body / numeric string Error — JavaScript function execution failed 12.0 (double)
args / empty array Error — $function 'args' must be an array NaN (double)
args / numeric array Error — $function 'args' must be an array 2.0 (double)
args / mixed array Error — $function 'args' must be an array 2.0 (double)
args / nested array Error — $function 'args' must be an array "1,21"

Download full comparison evidence.