server is directing to my c:/ directory instead of message

What I am trying to do is when the user clicks the calculate button(not doing any calculations yet) it should redirect them to a new screen with with a response message which says "Thanks for posting that!". But instead of the message it would display my entire C:\ directory, I feel like it has something to do with the "/" as the route but in the tutorial that what they used.

here's my index.html

<!DOCTYPE html>
<html lang="en" dir="ltr">
  <head>
    <meta charset="utf-8">
    <title>calculator</title>
  </head>
  <body>
    <h1>Calculator</h1>
    <form class="" action="/" method="post"> <!-- Send to home route-->
      <input type="text" name="num1" placeholder="First Number">
      <input type="text" name="num2" placeholder="Second Number">
      <button type="submit" name="submit">Calculate</button>
    </form>

  </body>
</html>

Here's the server calculator.js

const express = require("express");
const app = express();

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


app.post("/", function(req,res){
  res.send("Thanks for posting that!");
});


app.listen(3000,function(req,res){
  console.log("server up");
});