[chrismorgan]

The early DOM APIs certainly have some warts due to inheritance design mismatch, because they designed things in a single-inheritance way despite DOM actually being multiple-inheritancey. These days, new things like the append() method get added to the ParentNode interface, so that they’re provided by Element and DocumentFragment, but not such as Text or Comment; but back when they designed these things at the start, they only had Node, and decided to implement these methods on Node rather than individually on Element and DocumentFragment. While I imagine there were practical reasons they did it that way then, with the benefit of hindsight I state categorically that this was a mistake and is a design error. If the spec- and browser-makers were willing to break backwards compatibility, firstChild would certainly be shifted from Node to ParentNode, and text nodes would no longer have a firstChild member.

> There are so many languages which do just fine with one null type; […] two in JavaScript

I quibble over your claim here, because JavaScript doesn’t have two null types. undefined is not a null type. Rather, it’s JavaScript’s equivalent of what is in most dynamically-typed languages an exception, and a compilation error in most or all statically-typed languages. As a concrete example, where JavaScript produces undefined, Python raises AttributeError or KeyError. There are similarities to NaN as well—both come from a philosophy of trying to keep the code running even if you ask for something that isn’t defined. Yes, undefined then behaves a lot like null in some ways, just as NaN behaves like a number in some ways (though the two cases are not parallel, merely passingly similar). But then, false behaves like null in most of the same ways too (though not quite all—most notably, the `== null` comparison—but it’s well understood that a lot of JavaScript’s == comparisons are illogical and inconsistent, so this should not be considered significant to the design); yet no one claims false to be a third null value.