var coll = document.getElementsByClassName("collapsible");
var i;

for (i = 0; i < coll.length; i++) {
  coll[i].addEventListener("click", function() {
    this.classList.toggle("active");
    var content = this.nextElementSibling;

    if (content.style.display === "block") { content.style.display = "none";
    } else { content.style.display = "block"; }

  });
}