JSFiddle - React, Tailwind, and code Playground

HTML

<form id="form" action="">
  <label for="numbers">Zadejte celá čísla oddělená čárkou:</label>
  <input type="text" id="numbers">
  <button id="calc">Zjisti číslo</button>
  <p>Výsledné číslo: <output id="result">?</output></p> 
</form>

JavaScript

document.querySelector("#form").addEventListener("submit", function (event) {
	
  event.preventDefault();
  
  var input = (document.querySelector("#numbers").value || "0").split(","),
    min = Math.min(...input),
    max = Math.max(...input);

  	document.querySelector("#result").innerHTML = Math.round(min + (Math.random() * (max - min)));
});