JSFiddle - React, Tailwind, and code Playground

HTML

<form id="sum">
  <input type="number" name="a">
  <input type="number" name="b">
  <button type="submit">Отправить</button>
</form>

JavaScript

let form = document.getElementById( "sum" );

form.addEventListener( "submit", ( e ) => {
	e.preventDefault();
  alert(
  	( +form.elements.a.value ) + ( +form.elements.b.value )
  );
});