If I use External JavaScript for Heading, Footing and Navigation so I can do site wide updates. How does that effect my web page as far as Search Engines reading my site? Such as Google Adsense and Search Engine Placement?

Thank you,
Kimber

Absolutely no good at all

SE crawlers and indexers don't "do" javascript

Use server side includes instead.

If you do a serverside include you might as well just put it in an external .js file. When the page gets loaded even by a bot the serverside include includes the file. So the bot won't see anything different than before. Having it in an external .js file is better than having all the code in the page itself. Leave it how it is.

Huh?

If the content is in an external .js it wont even get put onto the page when the page is indexed by the search engines. So how is it useful to leave it like it is ?

In my experience with my own sites, external code IS indexed adequately by crawlers, bots and indexers. A bot is, after all, in essence an automated page reader that reads the page just as if it were being read from a browser physically, after all, the bot does not reside on the server - it is, in essence, a client, and as such reads the page as would any other client. I might be wrong, but that's my understanding of it.

I seriously doubt your javascript is being indexed. Use server-side includes.

It depends. If you're using JavaScript to actually write content (or links, or anything you want the bots to find), then yes, you probably are damaging your search engine placement. Although (some) bots do have the ability to parse JavaScript, taking the google bot for example, it probably wouldn't bother. The vast majority of JavaScript code out there isn't going to be used to render content, so why waste resources sifting through it?
On the other hand, if the code in the external JavaScript file is doing things like image rollovers, etc, then it's probably a good thing.