You mean testing if something is an array by checking if it has an includes function? Array.isArray() has been around for longer and has much wider browser support. includes also exists on strings, so by testing if a thing has that function it could be either an array or a string.

With this approach, there is no active switch. If the operating system supports Dark Mode, the CSS respects that global setting. Like when I go into my iPhone’s settings and switch EVERYTHING to Dark Mode (Settings.app → Display & Brightness → Appearance). macOS has a similar setting. I don’t know where to find it on Android, Windows, or Linux. If we wanted an active toggle a visitor can use, we’d have to use JavaScript instead of (or possibly in addition to) this media query. You can see a demo of this by switching your phone from light to dark mode on twitter.com, they seem to use this method. I added it to my site just last week: islovely.co/posts