Store a JavaScript Function on the Server¶

Note

Do not store application logic in the database. There are performance limitations to running JavaScript inside of MongoDB. Application code also is typically most effective when it shares version control with the application itself.

There is a special system collection named system.js that can store JavaScript functions for reuse.

To store a function, you can use the db.collection.save(), as in the following examples:

copy

db.system.js.save(
   {
     _id: "echoFunction",
     value : function(x) { return x; }
   }
)

db.system.js.save(
   {
     _id : "myAddFunction" ,
     value : function (x, y){ return x + y; }
   }
);

The _id field holds the name of the function and is unique per database.
The value field holds the function definition.

Once you save a function in the system.js collection, you can use the function from any JavaScript context; e.g. $where operator, mapReduce command or db.collection.mapReduce().