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Evaluation Query Operators

$where

On this page

  • Definition
  • Behavior
  • Available JavaScript Properties and Functions
  • elemMatch
  • Considerations
  • JavaScript Enablement
  • Unsupported Array and String Functions
  • Example

Definition

$where

Use the $where operator to pass either a string containing a JavaScript expression or a full JavaScript function to the query system. The $where provides greater flexibility, but requires that the database processes the JavaScript expression or function for each document in the collection. Reference the document in the JavaScript expression or function using either this or obj .

{ $where: <string|JavaScript Code> }

Note

Starting in MongoDB 4.4, $where no longer supports the deprecated BSON type JavaScript code with scope (BSON Type 15). The $where operator only supports BSON type String (BSON Type 2) or BSON type JavaScript (BSON Type 13). The use of BSON type JavaScript with scope for $where has been deprecated since MongoDB 4.2.1.

Note

Aggregation Alternatives Preferred

The $expr operator allows the use of aggregation expressions within the query language. And, starting in MongoDB 4.4, 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.

Behavior

Available JavaScript Properties and Functions

map-reduce operations and $where operator expressions cannot access certain global functions or properties, such as db, that are available in mongosh.

The following JavaScript functions and properties are available to map-reduce operations and $where operator expressions:

Available Properties
Available Functions
args
MaxKey
MinKey
assert()
BinData()
DBPointer()
DBRef()
doassert()
emit()
gc()
HexData()
hex_md5()
isNumber()
isObject()
ISODate()
isString()
Map()
MD5()
NumberInt()
NumberLong()
ObjectId()
print()
printjson()
printjsononeline()
sleep()
Timestamp()
tojson()
tojsononeline()
tojsonObject()
UUID()
version()

elemMatch

Only apply the $where query operator to top-level documents. The $where query operator will not work inside a nested document, for instance, in an $elemMatch query.

Considerations

  • Do not use global variables.

  • $where evaluates JavaScript and cannot take advantage of indexes. Therefore, query performance improves when you express your query using the standard MongoDB operators (e.g., $gt, $in).

  • In general, you should use $where only when you cannot express your query using another operator. If you must use $where, try to include at least one other standard query operator to filter the result set. Using $where alone requires a collection scan.

Using normal non-$where query statements provides the following performance advantages:

  • MongoDB will evaluate non-$where components of query before $where statements. If the non-$where statements match no documents, MongoDB will not perform any query evaluation using $where.

  • The non-$where query statements may use an index.

JavaScript Enablement

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

However, if you do not use these operations, disable server-side scripting:

See also ➤ Run MongoDB with Secure Configuration Options.

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.

For the complete list of removed array and string functions, see the 6.0 compatibility notes.

Example

Consider the following documents in the players collection:

db.players.insertMany([
   { _id: 12378, name: "Steve", username: "steveisawesome", first_login: "2017-01-01" },
   { _id: 2, name: "Anya", username: "anya", first_login: "2001-02-02" }
])

The following example uses $where and the hex_md5() JavaScript function to compare the value of the name field to an MD5 hash and returns any matching document.

db.players.find( { $where: function() {
   return (hex_md5(this.name) == "9b53e667f30cd329dca1ec9e6a83e994")
} } );

The operation returns the following result:

{
   "_id" : 2,
   "name" : "Anya",
   "username" : "anya",
   "first_login" : "2001-02-02"
}

As an alternative, the previous example can be rewritten using $expr and $function. Starting in MongoDB 4.4, you can define custom aggregation expression in JavaScript with the aggregation operator $function. To access $function and other aggregation operators in db.collection.find(), use with $expr:

db.players.find( {$expr: { $function: {
      body: function(name) { return hex_md5(name) == "9b53e667f30cd329dca1ec9e6a83e994"; },
      args: [ "$name" ],
      lang: "js"
} } } )

If you must create custom expressions, $function is preferred over $where.

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