simple javascript no library accordion

HTML

<button class="accordion">Section 1</button>
<div class="panel">
  <p>Lorem ipsum...</p>
</div>

<button class="accordion">Section 2</button>
<div class="panel">
  <p>Lorem ipsum...</p>
</div>

<button class="accordion">Section 3</button>
<div class="panel">
  <p>Lorem ipsum...</p>
</div>

CSS

/* Style the buttons that are used to open and close the accordion panel */
.accordion {
  background-color: #eee;
  color: #444;
  cursor: pointer;
  padding: 18px;
  width: 100%;
  text-align: left;
  border: none;
  outline: none;
  transition: 0.4s;
}

/* Add a background color to the button if it is clicked on (add the .active class with JS), and when you move the mouse over it (hover) */
.active, .accordion:hover {
  background-color: #ccc;
}

.accordion:after {
  content: '\02795'; /* Unicode character for "plus" sign (+) */
  font-size: 13px;
  color: #777;
  float: right;
  margin-left: 5px;
}

.active:after {
  content: "\2796"; /* Unicode character for "minus" sign (-) */
}

/* Style the accordion panel. Note: hidden by default */

.panel {
  padding: 0 18px;
  background-color: white;
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.2s ease-out;
}

Javascript

var acc = document.getElementsByClassName("accordion");
var i;

for (i = 0; i < acc.length; i++) {
  acc[i].addEventListener("click", function() {
    this.classList.toggle("active");
    var panel = this.nextElementSibling;
    if (panel.style.maxHeight) {
      panel.style.maxHeight = null;
    } else {
      panel.style.maxHeight = panel.scrollHeight + "px";
    }
  });
}