// connect to mongodb
var MongoClient = require('mongodb').MongoClient,
f = require('util').format;

var user = encodeURIComponent('admin'),
password = encodeURIComponent('123456'),
authMechanism = 'DEFAULT',
authSource = 'admin';

// connection url
var url = f('mongodb://%s:%s@localhost:27017?authMechanism=%s&authSource=%s',
user, password, authMechanism, authSource);

var db = null;
MongoClient.connect(url, function(err, db) { //Here is line 20!
    if(err) {
        console.log('Unable to connect to the mongoDB server. Error:', err);
    }
    else {
        console.log('Connection established to', url);
        db = database // once connected, assign the connection to the global variable
    }
})

Error: missing delimiting slash between hosts and options.
at module.exports (/home/lixing/Dropbox/thesis/node_modules/mongodb/lib/url_parser.js:37:11)
at connect (/home/lixing/Dropbox/thesis/node_modules/mongodb/lib/mongo_client.js:289:16)
at Function.MongoClient.connect (/home/lixing/Dropbox/thesis/node_modules/mongodb/lib/mongo_client.js:113:3)
at Object.<anonymous> (/home/lixing/Dropbox/thesis/server.js:20:13)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
at Function.Module._load (module.js:438:3)
at Module.runMain (module.js:604:10)