javascript - Cannot call method 'find' of undefined when running in nodejs -

i'm using mongodb(mongojs) , nodejs. when run server.js following error. typeerror: cannot call method 'find' of undefined

i not know why happen.this server.js:

var express = require('express'); var app = express(); var server = require('http').server(app); var io = require('socket.io')(server)  // var mongojs = require('mongojs'); // var db = mongojs('candidates', ['candidates']);  var databaseurl = "http://localhost:27017/candidates";  var db = ('candidates', ['candidates']); var mongojs = require('mongojs').connect(databaseurl, db);   var bodyparser = require('body-parser');  app.use(express.static(__dirname + "/public")); app.use(bodyparser.json());    io.sockets.on("connection", function(socket) {       console.log('user connected');       socket.on("disconnect", function(o) {          console.log('user left');       });       socket.emit("connected",{message:'helloworld'});       socket.on("addvote",function(vote){          console.log('adding vote');          console.log(vote);          io.sockets.emit("addvote",vote);       })         socket.on("unvote",function(vote){          console.log('unvote');          console.log(vote);          io.sockets.emit("unvote",vote);       })        socket.on("getprecinct",function(no){          console.log('getprecinct');          console.log(no);          io.sockets.emit("getprecinct",no);       })        socket.on("nextballot",function(add){          console.log('nextballot');          console.log(add);          io.sockets.emit("nextballot",add);       })    })     app.get('/candidates', function (req, res) {     console.log('i received request');      db.candidates.find(function (err, docs) {       console.log(docs);       res.json(docs);     });   });     app.put('/updatevotes/:precint_id', function (req, res) {     var id = req.params.precint_id;     db.candidates.findandmodify({       query: {precint_id:id },       update: {$set: {votes: req.body}},       new: true}, function (err, doc) {         console.log(err);         if(doc == null){           db.candidates.save({precint_id:id,votes:req.body})         }       }     );   });    server.listen(3000);   console.log("server running on port 3000"); 

the error happens when add variable databaseurl. add because if have 3 computers want them connect 1 computer server mongodb installed. why error of cannot call method 'find' of undefined . missing? help. thank you