Using express to develop a socket.io chat server in node.js, the problem of file path in app.js

Terminal error: Path must be a string. Received null

it feels like a problem with _ _ dirname, but I still reported an error after I changed it to the relative path. I would like to ask the great god to guide the rookies who have just learned node. Thank you very much.

under app.js:

var app = require("express").createServer(),
    io = require("socket.io").listen(app);

app.listen(1111);

app.get("/",function(req,res){
    res.sendfile(__dirname+"/index.html");

});

io.sockets.on("connection",function(socket){
    socket.emit("welcome",{text:"hello world!"})
})

under index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>socket.io  chatting!</title>
    <script src="/socket.io/socket.io.js"></script>
</head>
<body>
    <h1>let"s chatting~</h1>
    <script>
     var socket = io.connect();
     socket.on("welcome",function(data){
         console.log(data.text);
     })
    </script>
</body>
</html>