Can I load javascript code using <link> tag?

No. There was a proposal to allow:

<link rel="script" href=".../script.js"/>

analogously to stylesheets. This is even quoted as an example in the HTML 4 DTD, but browser implementation never happened. Shame, as this would have been much cleaner.

You need to use the <script> tag to include JavaScript source files:

<script type="text/javascript" src="mysrc.js"></script>

The end tag must be the full </script>, don't abbreviate the way you can with some tags as in <script type="text/javascript" src="..."/>.

Yes, alert statements in the included source will appear when they are evaluated by the browser.

For information on the uses of the <link> tag, see w3.org.