我用nodeJS创建了一个mongoDB表单，我可以提供signUp页面，但是当我单击任何其他链接转到其他页面时，例如会抛出“ Cannot GET /index.html”。我不确定如何使用nodejs服务多个文件。

var express = require("express");
var app = express();
var port = 3000;
var bodyParser = require('body-parser');
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));

var mongoose = require("mongoose");
mongoose.Promise = global.Promise;
mongoose.connect("mongodb://localhost:27017/ClubArchive_Users");
var nameSchema = new mongoose.Schema({
     fname: String,lname: String,email: String,uname: String,pwd :String
});
var User = mongoose.model("User",nameSchema);

app.get("/",(req,res) => {
    res.sendFile(__dirname + "/SignUp.html");
});

app.post("/addname",res) => {
    var myData = new User(req.body);
    myData.save()
        .then(item => {
            res.sendFile(__dirname + "/index.html");
        })
        .catch(err => {
            res.status(400).send("Unable to save to database");
        });
});

app.listen(port,() => {
    console.log("Server listening on port " + port);
});