JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>js test</title>
    <script src="script.js"></script>
</head>
<body>
	<p>Введите число: <input type="text" id="n1"></p>
	<p>Введите число: <input type="text" id="n2"></p>
	<button onclick="calc()" data-type='plus'>Прибавить</button>
	<button onclick="calc()" data-type='minus'>Отнять</button>
	<hr>
	<p id="result"></p>
</body>
</html>

JavaScript

function calc() {
  var num1, num2, result, button, type;
  
  button = event.srcElement;
  type = button.getAttribute('data-type');
  
  result = document.getElementById('result');
  
  num1 = parseInt(document.getElementById('n1').value); 
  num2 = parseInt(document.getElementById('n2').value);
  
  if(!num1 || !num2) {
    result.innerHTML = ('Введите число!');
  } else {
    if (type === 'plus') result.innerHTML = num1 + num2;
    else result.innerHTML = num1 - num2;
  } 
}