Why we should avoid use of Java scripts?

[Chris]

It depends on what JavaScript does on the page. If it is basically building the site, that's bad.
The reason is that SEO depends on a page being more or less in its final form even if JavaScript were not activated in the browser, because the robots that crawl pages don't run JavaScript functions, they just look at the raw HTML code they get from the server.

Imagine a page where at first, it displays only a loading screen, and using JavaScript, proceeds to load the page in the background, then removes the loading screen. All a robot would see is the loading screen, no other data, no content, nothing.

But a generic statement like "for SEO, you should avoid JavaScript" is much too broad (as long as, like I said, JavaScript is mostly used for visual effects and the like).