Why can not my client get socket.io.js? - node.js

Question

I am learning to use socket.io, but when I add
script(type='text/javascript', src='/socket.io/socket.io.js')
in my client, and I add code:
var http = require('http').Server(app);
var io = require('socket.io')(http);
io.on('connection', function(socket){
console.log('------------connection');
});
in app.js, but my client do get 404 error when trying to get /socket.io/socket.io.js.
I chage my code to :
var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
var routes = require('./routes/index');
var users = require('./routes/users');
var app = express();
var server = require('http').createServer();
var io = require('socket.io').listen(server);
// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');
// uncomment after placing your favicon in /public
//app.use(favicon(__dirname + '/public/favicon.ico'));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));
app.use('/', routes);
app.use('/users', users);
var userlist = {};
io.on('connection', function(data){
console.log('------------connection');
socket.name = data.user;
if (!userlist[data.user]) {
userlist[data.user] = data.user;
};
io.emit("online", { userlist: userlist, user: data.user});
});
// catch 404 and forward to error handler
app.use(function(req, res, next) {
var err = new Error('Not Found');
err.status = 404;
next(err);
});
// error handlers
// development error handler
// will print stacktrace
if (app.get('env') === 'development') {
app.use(function(err, req, res, next) {
res.status(err.status || 500);
res.render('error', {
message: err.message,
error: err
});
});
}
// production error handler
// no stacktraces leaked to user
app.use(function(err, req, res, next) {
res.status(err.status || 500);
res.render('error', {
message: err.message,
error: {}
});
});
server.listen(app.get('port'), function(){
console.log('Express server listening on port ' + app.get('port'));
});
module.exports = app;
and I also have the code as follow in my javascript:
$(document).ready(function() {
var socket = io.connect('http://localhost:3000');
// other code
}