JSFiddle - React, Tailwind, and code Playground

HTML

<p>Товар один <button class="text123" data-num = "111">Купить</button></p>
	<p>Товар два <button class="text1" data-num = "222">Купить</button></p>
    <p>Товар три <button class="text2" data-num = "333">Купить</button></p>

JavaScript

var button = document.getElementsByTagName('button');
for (var i = 0; i < button.length; i++) {
    var btn = button[i];
    btn.onclick = function(){
        alert(btn.getAttribute('data-num'));
    }
};