[chrismorgan]

Element.foo being undefined says “foo? What’s foo? I don’t know what you’re talking about.”

Element.{firstChild, nextSibling} being null means “I know what firstChild (or nextSibling) is, but this element doesn’t have one.”

It would be categorically wrong for things like Element.{firstChild, nextSibling} to be undefined.

[bikeshaving]

Some developers would interpret your rule as being, if it doesn’t make sense for you to make this property access on this specific object, the value should resolve to undefined and not null. Problem is, there already are mechanics for a kind of check like you describe (the in operator and hasOwnProperty both kinda do what you’re looking for). Moreover, if you were to design the DOM API from scratch, by your logic some developers might assume that text nodes, which never have children, should have a firstChild set to undefined and not null, because it should say “I don’t know what you’re talking about.” You can debate this last point, but know that some people are going to interpret the null/undefined rules as you describe in this way.

I’ve seen null/undefined semantics play out; they more or less collapse under the slightest of deadline pressures or scaling of team sizes. There are infinite ways to splice the idea of “emptiness” or “lack of data” and any definition you provide (the definitions both you and OP provide give a lot of wiggle room) would be interpreted fifteen ways by fifteen developers, so much so that even changing hands just once will cause any null/undefined distinctions to sublimate in any codebase.

There are so many languages which do just fine with one null type; I don’t understand why JavaScript developers have to do these sorts of mental kinesthetics to justify the existence of two in JavaScript, especially given the more or less insane fact that `typeof null === "object"`. If you consistently use undefined or null exclusively internally, and accept both via the == null trick externally, you can pretty much avoid having to deal with null/undefined semantics entirely.

[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.

[Jasper_]

Why does Array.prototype.find() return undefined and not null if it didn't find a value? According to your logic, the sentinel to indicate "this array doesn't have an element that matches the predicate" should be null.

[chrismorgan]

I don’t know. I would guess that that return type involved a slightly heated discussion before undefined won over null so that you can distinguish it returning an element that is null.

(This is a fundamental problem of nullable types which is solved by algebraic data types; in Rust, the equivalent method would return None for “no value found”, or Some(None) if it found a None value.)