We've generally adopted strict mode. I think it is now time to come up with and adopt a new no-undefined mode. And yes, it should break the standard library. It is worth it.
And on the other hand sql made the opposite mistake where NULL means both "this field has an empty value" and "this field does not have a value"
https://medium.com/@hbarcelos/why-i-banned-null-from-my-js-c...
Brendan Eich has elaborated on why the decision was made, whether it was a good one or not - null is an empty object pointer.
Idiomatically, I'd argue that `null` shouldn't exist in Javascript, since undefined is the global "default empty" value.
It's not very common but not unique either. Perl has `undef` that roughly works identically, which became generalized into the "undefinedness" concept in Raku [1]. Ruby doesn't have `undef` value, but it could have been in the alternative universe [2]. Even more languages have multiple absent values which are not necessarily compatible to the null-undef distinction (e.g. Objective-C).
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I think the separate `undef` value was mainly regarded as a solution to the apparent problem of detecting the absence in general, for example the absence of index or argument. Consider the following Python program for example:
def foo(obj=None): ...
_NOT_GIVEN = object()
def foo(obj=_NOT_GIVEN): ...
`Undef` might have been a working solution a decade ago, when we were still struggling with dynamically typed languages in general and systematic approaches were less common. Lua for example uses `nil` for both purposes; `t[key] = nil` is a valid way to remove given key from the table `t` (with a caveat that it doesn't shift any subsequent keys if the key was an integer) and an excess argument is filled with `nil` [3]. This is painful from time to time, say, a table of optional integers is not straightforward. `Undef` might have been a good compromise under this observation... if we didn't have any algebraic/sum data type like today.
[1] https://docs.raku.org/language/typesystem#Undefinedness
[2] https://stackoverflow.com/questions/6975266/what-is-the-unde...
[3] Lua even tried hard to remove any visible distinction between the actual `nil` and real absence of value! But it's still not perfect, and the discrepancy is much easier to detect from the C API.
None signifies absence, and if you need to pass a special value, then you might define a special object for that.
You can't distinguish `foo(obj=None)` from `foo()` because they are the same thing. You probably want something like `foo(obj=UNSET)`