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Server-side JavaScript

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.

Overview

MongoDB supports the execution of JavaScript code for the following server-side operations:

• mapReduce and the corresponding mongo shell method db.collection.mapReduce(). See Map-Reduce for more information.
• eval command, and the corresponding mongo shell method db.eval()
• $where operator
• Running .js files via a mongo shell Instance on the Server

JavaScript in MongoDB

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

See also

Store a JavaScript Function on the Server

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

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

You can run a JavaScript (.js) file using a mongo shell instance 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	copy db.adminCommand('listDatabases')
use <db>	copy db = db.getSiblingDB('<db>')
show collections	copy db.getCollectionNames()
show users	copy db.system.users.find()
show log <logname>	copy db.adminCommand({ 'getLog' : '<logname>' })
show logs	copy db.adminCommand({ 'getLog' : '*' })
it	copy cursor = db.collection.find() if ( cursor.hasNext() ){ cursor.next(); }

Concurrency

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

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