无法使用node.js连接到mongo副本

Question

var MongoClient = require('mongodb').MongoClient,
  assert = require('assert');
MongoClient.connect('mongodb://x.x.x:27017,x.x.x.x:27017/xx?replicaSet=xxx&wtimeoutMS=2000', function(err, db)            {
  // Create a collection we want to drop later
  var collection = db.collection('test');
  // Insert a bunch of documents for the testing
    collection.find().toArray(function(err, docs) {
      assert.equal(null, err);
//     test.equal(3, docs.length);
      console.log(docs);
      db.close();
    });
  });

    the error info :
TypeError: Cannot read property 'collection' of null
    at /Users/shanyy/LearningFiles/learnmongodb/test.js:7:22
    at connectCallback (/x/node_modules/mongodb/lib/mongo_client.js:315:5)
    at /x/node_modules/mongodb/lib/mongo_client.js:222:11
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)