TouchList is missing Symbol.iterator, can't use with 'for..of'

[fatcerberus]

TypeScript Version: 3.6.0-dev.20190608

Search Terms: touchlist iterator

Code

let touches = new TouchList();
for (const touch of touches) {

}

Expected behavior:
Code compiles successfully.

Actual behavior:
Error on line 2:
Type 'TouchList' must have a '[Symbol.iterator]()' method that returns an iterator. ts(2488)

Playground Link:
N/A because the Playground complains due to for...of requiring --downlevelIteration--but there's no way to enable it.

Related Issues:
#2695, labeled help wanted but closed

[jakebailey]

This is an old issue, but is this actually an issue given we have:

TypeScript/src/lib/dom.iterable.generated.d.ts

Lines 308 to 310 in 405251e

	interface TouchList {
	[Symbol.iterator](): IterableIterator<Touch>;
	}

This was introduced by #24669, but that PR is a full year older than this issue, so I'm not sure what's going on here such that this error ever appeared.

(@fatcerberus I think you're still around, so I'm not gonna close this in case you recall what was actually going on and I'm just reproducing it wrong.)

[fatcerberus]

@jakebailey I don’t recall; it might be that I didn’t have dom.iterable in my tsconfig or something (it was only recently that I found out that even existed).

It also seems weird that the playground required downlevelIteration for the for..of loop, for that matter… maybe in 2019 it only supported ES5 emit or something.

[jakebailey]

Ah, gotcha, yeah, one has to specify DOM.Iterable, it seems (the playground must). I guess I'm not well-versed enough to understand why that's not included by default, or if we should be hinting to users that "maybe you forgot this". @sandersn may know better than me, I'm just going over old issues looking for already fixed stuff!

[sandersn]

I can't even repro this locally. What's the tsconfig look like? I tried target: "es5" and lib: ["dom", "es5"].

[jakebailey]

All I have is:

// @filename: tsconfig.json
{
    "compilerOptions": {
        "lib": ["es2016", "dom"],
        "target": "ES2015",
        "noEmit": true,
        "types": []
    }
}

// @filename: test.ts
let touches = new TouchList();
for (const touch of touches) {

}

[fatcerberus]

I don’t think I have the project I was testing this in anymore, so I haven’t tested locally yet, but it’s still reproducible in the playground at least:
https://www.typescriptlang.org/play?downlevelIteration=true&target=1&module=0#code/CYUwxgNghgTiAEYD2A7AzgF3hpBXMAFiGgFzwAqehAMgJaYDcAUAGZIzwAUy6WtWSFtipE0ASngBvAL5MgA

[sandersn]

Argh, I wrote for (const touch in touches). in

[sandersn]

Anyway, there's a fixed list of constructors (Promise, Symbol, etc) in isES2015OrLaterConstructorName and in getCannotFindNameDiagnosticForName, some other values, that trigger a specific error. I didn't see anything smarter than that.

I don't know why dom.iterable is broken out from dom either. It was from before my time helping with DOM types.

[jakebailey]

Concretely, my questions are just:

• Is this issue closable, given it works when you include DOM.iterable?
• What's the deal with DOM.Iterable anyway?
  • Probably, it's because the DOM types are not split apart by ES versions as there's no mechanism to do so?

[sandersn]

1. Yes, I guess so. We could do better on missing lib errors, but I'm not sure how (cheaply, at least).
2. I don't understand why iterable is the dividing line (or, a dividing line) though.

[fatcerberus]

I always assumed the reason dom.iterable was split out is because it involves Symbol.iterator, which can’t (to my knowledge) by polyfilled correctly in ES5.

[jakebailey]

That's my guess, yeah, it's just that unlike setting say, es6 or es2017 or something, it doesn't actually get enabled automatically...