Dynamically, weakly typed programming language initially hacked together by Brandon Eich in 10 days as a weird cross-bread between Scheme and Java syntax.
It can be used as a general purpose language, but the most common and most problematic and privacy-invading use is in your web browser.
As a user, I prefer to disable JS by default with uBlock Origin. If I decide that some site really deserve it, I can easily allow it on the site basis. A lot of sites is broken broken without JS, but the privacy, security & performance benefits are worth it.
If you're a developer, please stop fcking up the web, you're not impressing anyone. Starting your project as a SPA may be tempting and popular choice, but in the long term you'll just hurt your users and yourself.