<div id="container">
  Hello after
</div>
<button id="change">apply after</button>

div#container {
  width:300;
  height:200;
  border:1px solid black;
  text-align:center;
}

.apply::after {
  content:"❣";
  text-align:center;
  width:100%;
}

const container = document.getElementById("container");
const changeBtn = document.getElementById("change");
let isAfter = false;
changeBtn.addEventListener("click", ()=>{
  if (!isAfter) {
    container.classList.add("apply");
    isAfter = true;
  } else {
    container.classList.remove("apply");
    isAfter = false;
  }
})