Because Google doesn't read the code for the most part.

I've seen some javascript picked up in the Google Cache (text version) & still other javascript not picked up, so it all depends how you code the script.

Iframes, I've seen Google read them before, I have no idea why they read some & not others on that one. I've never really put much time into researching them. An iframe is a separate web page (page inside a page), still I've seen G pick up content in the past from an iframe.

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Anything that you will use in iframe and javascript will be omitted from the search engine results. Google will not crawl your site

Regarding JavaScript, it depends what you do with it. The search engine spiders will not run JS and so if you change important SEO parts of your page (i.e. page title) with JS then it will not get picked up.

For general features that make things nicer for a vistor such as popping up a social media share list, rotating a Twitter feed, showing an 'add comment' box, and so on it's fine. Neither good nor bad for SEO.