value of type never can be created using document.all

[tyilo]

🔎 Search Terms

• document.all
• HTMLAllCollection

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Common "Bugs" That Aren't Bugs

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.4.5#code/GYVwdgxgLglg9mABMAFANwFyLAUzTgJwEos04YATRAbwChEHEIEBnOAGxwDp24BzFAAMAygAs4IdhTAByKIgBGORAAc4LFjAWcMggDSI0RANy0AvrVrMwLeQA9EAXkQU4EEAFscYKFwCG7OymMMCIKFAAnio4cKEOjgmIMuAUOMAwuBQyRDT0jKh2JuZAA

💻 Code

function f(v: never): void {
    console.log(`Shouldn't be possible:`, v);
}

const x = document.all;
if (typeof x === 'undefined') {
    f(x);
}

🙁 Actual behavior

The code type checks and f is called at runtime even though it has a parameter of type never.

🙂 Expected behavior

The type of x should not be narrowed have type undefined when typeof x === 'undefined' has been established.
Instead it's type should be narrowed to undefined | HTMLAllCollection.

Additional information about the issue

No response

[snarbies]

That's not how type narrowing works. If the declared type is X and CFA can determine that the type is also Y, then the proper inference is that the type is X & Y, not X | Y. It must be both, not either/or.

And since we know that it is both HTMLAllCollection and undefined, the correct inference is never.

Or are you suggesting that the typing of Document.all be changed to undefined | HTMLAllCollection?

[fatcerberus]

@snarbies document.all is a weird animal: because of spec weirdness for the purpose of not breaking the web, typeof document.all === 'undefined' is true (as well as it being falsy and loosely equal to both null and undefined) despite it really being an HTMLAllCollection.

[tyilo]

@snarbies No, I'm saying that the given the information that typeof x === "undefined", the conclusion is only that x has type undefined | HTMLAllCollection and not that x has type undefined. This is because typeof document.all is "undefined" even though it has type HTMLAllCollection.

[fatcerberus]

fwiw I doubt TS is going to make a special case in the type checker specifically to account the weirdness of document.all. This situation can't happen with any other value in JS and document.all is deprecated anyway; MDN recommends against using it at all.

[tyilo]

Yeah, I don't think this will be fixed. However I think it is nice to at least have an issue for every soundness issue.

[snarbies]

I'm not sure I would even characterize this as a soundness issue. It's closer to magic.

There are scenarios that the type system simply can not convey, and this is true of most any type system. Magic, including exotics, by its very nature can and will defy the type system.

The only fix here would be to introduce type magic around the HTMLAllCollection, which we can pretty safely assume wouldn't be happening.

[guillaumebrunerie]

A better fix would be to remove the type definition for document.all entirely. Why would anyone use it in a Typescript project?
The only reason it still exists is to avoid breaking antiquated websites, but they most likely don't use Typescript anyway (or are not being maintained anymore).

[RyanCavanaugh]

document.all is already marked deprecated.

[typescript-bot]

This issue has been marked as "Won't Fix" and has seen no recent activity. It has been automatically closed for house-keeping purposes.