Action when element exists

index.html

<div id="div1" class="select"><p>Mensagem de Sucesso</p></div>

<button id="div2" class="select"><p>Comprar algo</p></button>

makeExists.js

var div1 = document.getElementById("div1");
var div2 = document.getElementById("div2");

window.dataLayer = [];

var makeExists = function(el){
  el.onclick = function(){
    div1.style.display = "block";
  }; 
}

makeExists(div2);

var observer = new MutationObserver(function(mutations) {
  mutations.forEach(function(mutation) {
    if(mutation.attributeName === 'style'){
      dataLayer.push("Deu pra entender")
      console.log(dataLayer);
    }
  });    
});

// Notify me of style changes
var observerConfig = {
    attributes: true, 
  attributeFilter: ["style"]
};
observer.observe(div1, observerConfig);

style.css

#div1 {
  display: none;
}

Simple POC