rel nofollow

Hello!

How to make external links

"nofollow" with JQuery?

I want to attribute "rel" add "nofollow" like this

<a href="http://www.externalsite.com" rel="nofollow">do not follow</a>

but my domain this may not apply

<a href="http://www.mysite.com" rel="">yes, follow</a>

Thank you!