They don't block JavaScript, they block the loading of resources deemed to be for advertising. These are typically ads, but they can also be images, CSS, or other resources hosted by advertisers. Such blocking will also block any other kind of tracking done by the external domain.
On the other hand, JavaScript inline in the page is almost never blocked by users.
According to this random first result on Google 42.7% of internet users use ad blockers (which is what would usually block the js), and the number increases the techier the audience is.
Browser addons, rss readers, cli tools like youtube dl, light clients like chat thumbnailers, reader modes and so on don't either. This can add up depending of your site, and have a different significance than scrappers.