MongoDB - How can i get the size of a collection using Node.js?

I'm writing a server side code using node.js, i'm trying to get the the MongoDB collection size using count method which is not working. This is my code

    var mongo = require('mongodb');
var host =  "127.0.0.1";
var port = mongo.Connection.DEFAULT_PORT;

function getStock(name, callback) {

    var db = new mongo.Db("testDB", new mongo.Server(host, port, {}));
    db.open (function(error){
        console.log("We are connected! " + host + ":" +port);

        db.collection("stocks", function(error, collection){
            console.log("We have a collection");
            **var numOfDocs = db.collection('stocks').count()**
            **console.log("The num of  Docs in our collection is: ",numOfDocs)**
            collection.find({"name":name.toString()}, function(error, cursor) {
                cursor.toArray(function(error, stocks) {
                    if (stocks.length==0) {
                        //console.log("No Stocks found!");
                        callback(false);
                    }
                    else {
                        callback(stocks[0]);
                        //console.log("Found a stock -> ",stocks[0]);
                    }
                });
            });


        });
    });
}