// Get all elements with class="closebtn"
var close = document.getElementsByClassName("closebtn");
var i;
// Loop through all close buttons
for (i = 0; i < close.length; i++) {
  // When someone clicks on a close button
  close[i].onclick = function(){
    // Get the parent of <span class="closebtn"> (<div class="alert">)
    var div = this.parentElement;
    // Set the opacity of div to 0 (transparent)
   div.style.opacity = "0";
    // Hide the div after 600ms (the same amount of milliseconds it takes to fade out)
   setTimeout(function(){ div.style.display = "none"; }, 600);
 }
}