app.py

from flask import Flask, render_template, request, jsonify


app = Flask(__name__)


@app.route("/")
def home():
    return render_template("index.html")


@app.route("/square/", methods=["POST"])
def square():
    try:
        n = int(request.form.get("n", 0))
    except ValueError:
        return jsonify({"error": "provide a valid number for n"}), 400
    return jsonify({"n_squared": str(n * n)})

index.html

<h2>Arbitrarily Precise Square Calculator</h2>
<form id="squareForm">
<label for="n">number: </label>
<input type="number" name="n" id="n">
<button type="submit">submit</button>
</form>

<label for="result">result: </label>
<input placeholder="result" type="number" id="result" readonly>

<script>
const numEl = document.querySelector("#n");
const resultEl = document.querySelector("#result");
const squareForm = document.querySelector("#squareForm");

const handleSubmit = async function(e) {
  e.preventDefault();

  const payload = new FormData();
  payload.set("n", numEl.value);

  const resp = await fetch("/square/", {
    method: "POST",
    body: payload,
  });

  if (!resp.ok) alert("something went wrong!");
  else {
    const data = await resp.json();
    resultEl.value = data.n_squared;
  }
};

squareForm.addEventListener("submit", handleSubmit);
</script>