gracefully shutdown http server - node.js

Question

I write a http + websocket(socket.io) server, and catch SIGINT to stop it.
But I can't stop it gracefully. eg:
in a konsole window, I typed in:
>
xxx ~$ npm start
info - socket.io started
server started at :80
new web socket connection !
^C ### PRESS CTRL+C HERE
xxx ~$ ^C--- SIGINT RECIVED --- # <-- WTF??!! ( I have already been dropped to shell
xxx ~$ ps -f
UID PID PPID C STIME TTY TIME CMD
GongT 18776 2281 0 17:37 pts/11 00:00:00 /bin/bash
GongT 31365 1 0 21:24 pts/11 00:00:00 node main.js
GongT 31548 18776 0 21:24 pts/11 00:00:00 ps -f
# ^ look at PPID of node process
xxx ~$ kill -SIGINT 31365
--- SIGINT RECIVED ---
xxx ~$ kill -SIGINT 31365
--- SIGINT RECIVED ---
xxx ~$ # now I disconnect from client(web browser)
server stopped. bye~
My signal handler like that:
process.on('SIGINT', function (){
console.log(' --- SIGINT RECIVED --- ');
if(isServerRunning){
server.close();
isServerRunning = false;
setInterval(function(){
if(checkEveryThingFinished()){
console.log('server stopped. bye~');
process.exit();
}
});
}
});
My question is :
why I was dropped to shell without end the process, and how to prevent this?
(just waitting until process.exit())