OPTIONS

Server-side JavaScript

Overview

MongoDB provides the following commands, methods, and operator that perform server-side execution of JavaScript code:

  • mapReduce and the corresponding mongo shell method db.collection.mapReduce(). mapReduce operations map, or associate, values to keys, and for keys with multiple values, reduce the values for each key to a single object. For more information, see Map-Reduce.
  • $where operator that evaluates a JavaScript expression or a function in order to query for documents.

You can also specify a JavaScript file to the mongo shell to run on the server. For more information, see Running .js files via a mongo shell Instance on the Server

JavaScript in MongoDB

Although these methods use JavaScript, most interactions with MongoDB do not use JavaScript but use an idiomatic driver in the language of the interacting application.

You can also disable server-side execution of JavaScript. For details, see Disable Server-Side Execution of JavaScript.

Running .js files via a mongo shell Instance on the Server

You can specify a JavaScript (.js) file to a mongo shell instance to execute the file on the server. This is a good technique for performing batch administrative work. When you run mongo shell on the server, connecting via the localhost interface, the connection is fast with low latency.

The command helpers provided in the mongo shell are not available in JavaScript files because they are not valid JavaScript. The following table maps the most common mongo shell helpers to their JavaScript equivalents.

Shell Helpers JavaScript Equivalents
show dbs, show databases
db.adminCommand('listDatabases')
use <db>
db = db.getSiblingDB('<db>')
show collections
db.getCollectionNames()
show users
db.getUsers()
show roles
db.getRoles({showBuiltinRoles: true})
show log <logname>
db.adminCommand({ 'getLog' : '<logname>' })
show logs
db.adminCommand({ 'getLog' : '*' })
it
cursor = db.collection.find()
if ( cursor.hasNext() ){
   cursor.next();
}

Concurrency

Changed in version 2.4.

The V8 JavaScript engine, which became the default in 2.4, allows multiple JavaScript operations to execute at the same time. Prior to 2.4, MongoDB operations that required the JavaScript interpreter had to acquire a lock, and a single mongod could only run a single JavaScript operation at a time.

Refer to the individual method or operator documentation for any concurrency information. See also the concurrency table.

Disable Server-Side Execution of JavaScript

You can disable all server-side execution of JavaScript, by passing the --noscripting option on the command line or setting security.javascriptEnabled in a configuration file.

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