$function (expression operator)

Schema Validation Restriction

You cannot use $function as part of a schema validation query predicate.

Javascript Enablement

To use $function, you must have server-side scripting enabled (default).

If you do not use $function (or $accumulator, $where, or mapReduce), disable server-side scripting:

• For a mongod instance, see security.javascriptEnabled configuration option or --noscripting command-line option.

• For a mongos instance, see security.javascriptEnabled configuration option or the --noscripting command-line option.

  In earlier versions, MongoDB does not allow JavaScript execution on mongos instances.

See also ➤ Run MongoDB with Secure Configuration Options.

Alternative to $where

The query operator $where can also be used to specify JavaScript expression. However:

• The $expr operator allows the use of aggregation expressions within the query language.

• The $function and $accumulator allows users to define custom aggregation expressions in JavaScript if the provided pipeline operators cannot fulfill your application's needs.

Given the available aggregation operators:

• The use of $expr with aggregation operators that do not use JavaScript (i.e. non-$function and non-$accumulator operators) is faster than $where because it does not execute JavaScript and should be preferred if possible.

• However, if you must create custom expressions, $function is preferred over $where.

Unsupported Array and String Functions

MongoDB 6.0 upgrades the internal JavaScript engine used for server-side JavaScript, $accumulator, $function, and $where expressions and from MozJS-60 to MozJS-91. Several deprecated, non-standard array and string functions that existed in MozJS-60 are removed in MozJS-91.

Example 1: Usage Example

Create a sample collection named players with the following documents:

db.players.insertMany([
   { _id: 1, name: "Miss Cheevous",  scores: [ 10, 5, 10 ] },
   { _id: 2, name: "Miss Ann Thrope", scores: [ 10, 10, 10 ] },
   { _id: 3, name: "Mrs. Eppie Delta ", scores: [ 9, 8, 8 ] }
])

The following aggregation operation uses $addFields to add new fields to each document:

• isFound whose value is determined by the custom $function expression that checks whether the MD5 hash of the name is equal to a specified hash.

• message whose value is determined by the custom $function expression that format a string message using a template.

db.players.aggregate( [
   { $addFields:
      {
        isFound:
            { $function:
               {
                  body: function(name) {
                     return hex_md5(name) == "15b0a220baa16331e8d80e15367677ad"
                  },
                  args: [ "$name" ],
                  lang: "js"
               }
            },
         message:
            { $function:
               {
                  body: function(name, scores) {
                     let total = Array.sum(scores);
                     return `Hello ${name}.  Your total score is ${total}.`
                  },
                  args: [ "$name", "$scores"],
                  lang: "js"
               }
            }
       }
    }
] )

The operation returns the following documents:

{ "_id" : 1, "name" : "Miss Cheevous", "scores" : [ 10, 5, 10 ], "isFound" : false, "message" : "Hello Miss Cheevous.  Your total score is 25." }
{ "_id" : 2, "name" : "Miss Ann Thrope", "scores" : [ 10, 10, 10 ], "isFound" : true, "message" : "Hello Miss Ann Thrope.  Your total score is 30." }
{ "_id" : 3, "name" : "Mrs. Eppie Delta ", "scores" : [ 9, 8, 8 ], "isFound" : false, "message" : "Hello Mrs. Eppie Delta .  Your total score is 25." }

Example 2: Alternative to $where

As an alternative to a query that uses the $where operator, you can use $expr and $function. For example, consider the following $where example.

db.players.find( { $where: function() {
   return (hex_md5(this.name) == "15b0a220baa16331e8d80e15367677ad")
} } );

The db.collection.find() operation returns the following document:

{ "_id" : 2, "name" : "Miss Ann Thrope", "scores" : [ 10, 10, 10 ] }

The example can be expressed using $expr and $function:

db.players.find( {$expr: { $function: {
      body: function(name) { return hex_md5(name) == "15b0a220baa16331e8d80e15367677ad"; },
      args: [ "$name" ],
      lang: "js"
} } } )