JSFiddle - React, Tailwind, and code Playground

HTML

<div>
  Введите число: <input id="number" type="text">
  <button id="btn">Квадрат числа</button>
</div>

JavaScript

var num = document.getElementById("number").value;
var btn = document.getElementById("btn");
 
btn.addEventListener("click", function getSquareNumber() {
  alert(num*num);
});