Modal box on the node js - node.js

Question

I have the file upload api, which send out the SUCCESS message on the file upload and the FAILURE message when the file not selected.
app.js
const express = require("express");
const app = express();
const http = require("http").Server(app).listen(3000);
const upload = require("express-fileupload");
const request = require("request");
app.use(express.urlencoded());
app.use(express.json());
app.use(upload({
useTempFiles: true,
tempFileDir: '/tmp/'
}));
console.log("Server Started");
// To load both HTML and CSS files on the root call
app.use(express.static(__dirname + "/public"));
app.get("/", function (req, res) {
res.sendFile(__dirname + "/index.html");
}
)
app.post("/", function (req, res) {
if (req.files.filename.name) {
res.send("uploaded");
}
else
res.send("Please select the file to upload!");
})
Instead res.send(), i want to display some modal box defined on my html code. Need a help on how to incorporate or shown this modal box upon my post request.
index.html
<link rel="stylesheet" href="index.css">
<div class="hdngTab">
<h1 style="align-content: center"> Utility</h1>
</div>
<div class="test">
<form label="upload" method="post" enctype="multipart/form-data" action="/">
<div class="sf-ref">
<label> Ticket Reference </label>
<input type="text" name="sftext"> </input>
</div>
<br><br>
<input style="font-size: 20px; padding-left: 180px" type="file" name="filename">
<!--input style="font-size: 20px" type="submit" value="Upload"-->
<button class="trigger" style="font-size: 20px" value="Upload">
Upload </button>
</form>
</div>
<div class="modal">
<div class="modal-content">
<span class="close-button">×</span>
<h1>Hello, I am a modal!</h1>
</div>
</div>