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(). For more information, see Map-Reduce.

• $where operator that evaluates a JavaScript expression or a function in order to query for documents.

• $accumulator and $function aggregation operations that allows users to define custom aggregation expressions.

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

If you do not need to perform server-side execution of JavaScript code, 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.

Concurrency

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:

• For a mongod instance by passing the --noscripting option on the command line or setting security.javascriptEnabled to false in the configuration file.

• Starting in MongoDB 4.4, for a mongos instance by passing the --noscripting option on the command line or setting security.javascriptEnabled to false in the configuration file.

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