my project directory is -

app
├── app.js
├── index.html
├── node_modules
├── package.json
├── package-lock.json
├── signup_success.html
└── style.css

my app file code are given below-

app.js file code:-

var express=require("express");
var bodyParser=require("body-parser");

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/gfg');
var db=mongoose.connection;
db.on('error', console.log.bind(console, "connection error"));
db.once('open', function(callback){
    console.log("connection succeeded");
})

var app=express()


app.use(bodyParser.json());
app.use(express.static('public'));
//app.use(express.static(__dirname + "/../public"));
//app.use(app.router);
app.use(bodyParser.urlencoded({
    extended: true
}));

app.post('/sign_up', function(req,res){
    var name = req.body.name;
    var email =req.body.email;
    var pass = req.body.password;
    var phone =req.body.phone;

    var data = {
        "name": name,
        "email":email,
        "password":pass,
        "phone":phone
    }
db.collection('details').insertOne(data,function(err, collection){
        if (err) throw err;
        console.log("Record inserted Successfully");
            
    });
        
    return res.redirect('signup_success.html');
})


app.get('/',function(req,res){
res.set({
    'Access-control-Allow-Origin': '*'
    });
return res.redirect('index.html');
}).listen(3000)


console.log("server listening at port 3000");

when I am running the app.js file then I am getting "Cannot GET /index.html" on the browser? enter image description here