Javascript Detect Click event outside of div

Javascript Detect Click event outside of div

I have a div with id content-area, when some one click outside of that
div, i would like to alert user that you clicked outside of content area.
I don't know how can i do it in JavaScript. Please help me to solve this
issue. You help is much appreciated. Thanks in advance.
<div id = "outer-container">
<div id = "content-area">
Display Conents
</div>
</div>