CSS style to inline style via JavaScript

I want to add all CSS styles of a specific element to its inline style attribute. For example:

I have:

<div id="d"></div>

and:

#d { background: #444444; width: 50px; height: 20px; display: inline-block; }

Now I want a JavaScript function that turns my div into this:

<div id="d" style="background: #444444; width: 50px; height: 20px; display: inline-block;"></div>

Please help me. And, by the way, I don't want any CSS styles to re-write any existing inline style.