Copy highlighted code to the clipboard

Allowing users to copy sample code is a common thing in the web development nowadays. To demonstrate the implementation, we create two elements:
<pre id="sampleCode"><code>...</code></pre>
<button id="copyButton">Copy</button>
The sampleCode element contains the sample code which is already highlighted.
Copying the sample code to the clipboard could consist of three steps:
So we have to backup the current selected text before copying, and restore it at the end.
You can see the code in the demo below. Enjoy!

Demo

Copy highlighted code to the clipboard

See also