Protip: never even mention undefined in your codebase. Erase it from your vernacular. If you ever need to pass or return nothing, you use null.
My explanation was that it signals intent to me, and is different from some property not being part of the expected object shape or not having been initialized because of some accident or logic failure.
Since then, I've sticked to it, and am "allowed" to use NULL ^^
It can lead to some annoying checks in TS for primitively-typed properties, so for these, I still allow explicit usage of undefined when it's simpler given the surrounding code.
But I agree with you in principle. Using "undefined" as a "second nullish value" and explicitly checking for it is a programming error.
When there's object/areay vs null/undefined, thankfully, the truthiness narrowing often allows me to interface with code relying on "undefined" without explicitly handling this "value" in my own parts of the code base :)
undefined camp:
- out of bounds array index access [1,2,3][42]
- non existent object key access {}.foo
- array.find(...) no result
- ...
null camp:
- document.querySelector(...) no result
- regex.match(...) no result
- ...
Could the language have done without both null and undefined? Definitely. But it's here and here to stay.
Though, it's not the only language with two nulls. Julia has nothing and missing, though their semantics are more well defined and with different behaviors than in JavaScript.
No: it's the type that has one inhabitant—it's not that type's fault that it appears as a default argument or var or was left out of JSON... So long as typeof null === "object", null is the absurd one
undefined
Of course you shouldn't do that, but I once encountered a library that behaved differently depending on whether an option in an options bag object was not present or explicitly set to undefined.
You can run into similar ugliness with function parameters, if you do evil things like using
arguments
All not things you should do, but taints the "purity" argument, doesn't it? :D