db.eval()

The db.eval() provides the ability to run JavaScript code on the MongoDB server. It is a mongo shell wrapper around the eval command. However, unlike the eval command, the db.eval() method does not support the nolock option.

The method accepts the following parameters:

Consider the following example of the db.eval() method:

db.eval( function(name, incAmount) {
            var doc = db.myCollection.findOne( { name : name } );

            doc = doc || { name : name , num : 0 , total : 0 , avg : 0 };

            doc.num++;
            doc.total += incAmount;
            doc.avg = doc.total / doc.num;

            db.myCollection.save( doc );
            return doc;
         },
         "<name>", 5 );

• The db in the function refers to the current database.
• "<name>" is the argument passed to the function, and corresponds to the name argument.
• 5 is an argument to the function and corresponds to the incAmount field.

If you want to use the server’s interpreter, you must run db.eval(). Otherwise, the mongo shell’s JavaScript interpreter evaluates functions entered directly into the shell.

If an error occurs, db.eval() throws an exception. Consider the following invalid function that uses the variable x without declaring it as an argument:

db.eval( function() { return x + x; }, 3 );

The statement will result in the following exception:

{
   "errno" : -3,
   "errmsg" : "invoke failed: JS Error: ReferenceError: x is not defined nofile_b:1",
   "ok" : 0
}