Why can't I just use "url(imageURL)" to set a background pictuer?
<pre><code>&lt;img class = "preview" alt = "Styling with a Bandana" 
src = "https://s3-us-west-2.amazonaws.com/s.cdpn.io/389177/bacon.jpg"
 onmouseover = "upDate(this)" onmouseout = "unDo()"&gt;

function upDate(previewPic) 
{
 imageURL = previewPic.src;
 document.getElementById("image").style.backgroundImage =  "url('" + imageURL + "')";
}
</code></pre>

<p>Why should I use <code>"url('" + imageURL + "')"</code> to set background image?
I had tried to use <code>"url(imageURL)"</code> but it couldn't work.</p>