Type inference with for...in

[johnfn]

I would expect the following code to typecheck and work:

const x = { a : 1, b : 2 };

for (const foo in x) {
    console.log(x[foo]);
}

However, it gives an error that x has no index signature.

The problem appears to be that foo is not inferred to be of type keyof (typeof x). If I do the following, there are no errors:

const x = { a : 1, b : 2 };

for (const foo in x) {
    console.log(x[foo as keyof (typeof x)]);
}

Shouldn't x be inferred to be that type?

(Apologies if this issue already exists somewhere. I searched pretty hard and I could not find it.)

---

It also seems weird that something like this infers the type of foo properly. We have less information about x, yet our type constraint on x is stricter.

function test<T>(x: T) {
    for (const foo in x) {
        console.log(x[foo]);
    }
}

[RyanCavanaugh]

#12314, #12988

[johnfn]

Ah, that's pretty tough for something which developers do all the time.

Are there any workarounds for iterating over and indexing an object in a typesafe manner? One I see is to give all my types index signatures, but I have a habit of defining object literals and allowing the type inference to take over.

[RyanCavanaugh]

any (non-strict) or { [k: string]: { } } (strict) are the types you should assert x to in this case.

Since we don't know (in general) that x doesn't actually reference a subtype with other keys of different types, there's no known-correct type for us to produce for x[k] in a for(k in x).