varapp=require("http").createServer(handler);vario=require("socket.io")(app);app.listen(8081);functionhandler(req,res){console.log("what??");res.writeHead(200);res.end("hello");}varapp=require("http").createServer(handler);vario=require("socket.io")(app);app.listen(8081);varhandler=function(req,res){console.log("what??");res.writeHead(200);res.end("hello");}第一段代碼的handler就可以用，第二段代碼的handler就不能用，誰能解釋下？