Edit in JSFiddle

var Smooth = {
    init: function(id) {
        var el = document.getElementById(id);
        if (!el.hasAttribute('data-height')) {
            el.style.display = 'block';
            el.dataset.height = el.offsetHeight+"px";
            el.className = 'smooth';
            el.style.height = 0;
        }
        return el;
    },
    expand: function(id) {
        var div = Smooth.init(id);
        setTimeout(function() { div.style.height = div.dataset.height; }, 20);
    },
    collapse: function(id) {
        var div = Smooth.init(id);
        div.style.height = 0;
    }
}
body { font: 14px sans-serif }

.smooth { transition: height .5s }

div {
  width:200px;
  background:red;
  color:white;
  display:none;
  overflow: hidden;
}
<p>
  <button type="button" onclick="Smooth.expand('div')">Open</button>
  <button type="button" onclick="Smooth.collapse('div')">Close</button>
</p>

<div id="div">
  Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.
</div>