nodejs+express+mongodb

var mongodb = require('mongodb');
var host = "127.0.0.1";
var port = 27017;
var server = new mongodb.Server(host, port,{safe:true});
var dbname = 'ppt';
var tablename = 'user';
var db = new mongodb.Db(dbname,server,{safe:true});

var DB = function(){};
DB.prototype.insert = function(json) {
db.open(function(){
db.collection(tablename, function(err, col){
col.insert(json,function(err, result) {
if(err) {
console.error(err.stack);
} else {
console.log('result is: ' + result);
return result;
}
});
});
});
};

DB.prototype.update = function(key,value) {
db.open(function(){
db.collection(tablename, function(err, col){
col.update(key, value,function(err, result) {
if(err) {
console.error(err.stack);
} else {
console.log('result is: ' + result);
return result
}
});
});
});
};

DB.prototype.delete = function(key) {
db.open(function(){
db.collection(tablename, function(err, col){
col.remove(key,function(err, result) {
if(err) {
console.error(err.stack);
} else {
console.log('result is: ' + result);
return result;
}
});
});
});
};

DB.prototype.select = function(param) {
db.open(function(){
db.collection(tablename,{safe:true}, function(err, col) {
console.log('collection');
col.find(param).toArray(function(err,result){
if(err) {
console.error(err);
} else {
console.log(result);
return result;
}
});
});
});
};

module.exports = new DB();