for .. in type is wrong for objects as const

[fabio-colella-md]

Bug Report

When declaring a new object as const, if we then use a for .. in structure, the type of the key is just inferred as string and not as the more restricted type that const defines. This causes issues especially when trying to use that key to get the corresponding property.

🔎 Search Terms

is:issue for in as const

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about const

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.8.0-dev.20220621#code/MYewdgzgLgBAtgTwMLmgeQEYCsYF4YDeAsAFAwwCGAXDAERQCm0AjLQDSnkY31NQBMtUgF9KEGKEhQA3KVIAzEACcYACknQYIbAGkGCGAEsw8ZKiiYsASkKdTKKZYDa2rHoQBdEaSA

💻 Code

const myConstObj = {
  a: "test1",
  b: "test2"
} as const;

for (const objKey in myConstObj) {
  myConstObj[objKey]
}/* ^^^^^^^^^^^^^^^^
    const objKey: string
    --------------------
    Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 
    '{ readonly a: "test1"; readonly b: "test2"; }'.
        No index signature with a parameter of type 'string' was found on type 
    '{ readonly a: "test1"; readonly b: "test2"; }'.(7053)
*/

🙁 Actual behavior

When using the key to get the prop, we are told that the prop would have type any because the key is of type string.

🙂 Expected behavior

No error should be reported, as the correct inferred type for the key should be the union of the keys (i.e. 'a' | 'b' in this case) of the const object.

🤔 Workaround

"Coerce" the key type to be what we would have expected, so in this case:

const myConstObj = {
  a: "test1",
  b: "test2"
} as const;

for (const _objKey in myConstObj) {
  const objKey = _objKey as keyof typeof myConstObj
  myConstObj[objKey]
}

[MartinJohns]

Duplicate of #44706.

[fcole90]

This issue is different, as this one is restricted only to objects as const 😉

[MartinJohns]

That makes no difference. Objects are not sealed.

const myConstObj = {
  a: "test1",
  b: "test2"
} as const;

type Data = {
    a: "test1";
    b: "test2";
    c?: number;
}

function foo(data: Data) { data.c = 123; }

foo(myConstObj);

for (const key in myConstObj) {
    // Will print out: a, b, c
    console.log(key);
}

The only case where this would work safely is if the object is used right after it's been declared, like in your example. But that's such a narrow corner case that I doubt the team will support it.

[RyanCavanaugh]

The feature needed to make this OK is exact types. as const changes the type of an expression, it's not something that's reflected in the type system per se. IOW there's nothing from TS's perspective that distinguishes the code in the OP from this code, which would violate the keyof assumption:

const myConstObjTmp = {
  a: "test1",
  b: "test2"
} as const;
const gotcha = { a: "test1", b: "test2", c: "test3" } as const;
const myConstObj: typeof myConstObjTmp = gotcha;

[fabio-colella-md]

I understand your points, indeed there are many valid scenarios where that type I suggested would not be correct 🤔
Hence, I think I should just keep using the workaround. Do you have any suggestions on making that cleaner and more concise? I think it's a very common scenario. I wanted to specify the type from within the for but I couldn't find a way 🤔 At least, I'd like to avoid the need of creating an extra variable 😅

I mean, this doesn't work, but something like this I think would be very useful:

const myConstObj = {
  a: "test1",
  b: "test2"
} as const;

for (const objKey (as keyof typeof myConstObj) in myConstObj) {
  myConstObj[objKey]
}

[fabio-colella-md]

I'm now also thinking, that you're right that the type I suggested is not correct in many scenarios, however, I still think there might be a narrower type. So, taking @MartinJohns's example, why can't the inferred type be 'a' | 'b' | 'c' instead of a very generic string? 🤔

[MartinJohns]

So, taking @MartinJohns's example, why can't the inferred type be 'a' | 'b' | 'c' instead of a very generic string?

Because the compiler doesn't know there's a property "c". It sees a type that has only two properties: "a" and "b".

[fabio-colella-md]

Mmh, interesting, thank you everybody for all your explanations 😊 I think this issue can be closed 😊

[fabio-colella-md]

Alternative workaround

const myConstObj = {
  a: "test1",
  b: "test2"
} as const;

for (const objKey of Object.keys(myConstObj) as (keyof typeof myConstObj)[]) {
  myConstObj[objKey]
}

[MartinJohns]

function getObjectKeysUnsafe<T extends object>(value: T): (keyof T)[] { return Object.keys(value) as (keyof T)[]; }

for (const objKey of getObjectKeysUnsafe(myConstObj)) {
  myConstObj[objKey]
}

[fabio-colella-md]

Nice, thanks 😊

[Darrick-Oliver]

I found that using a for..of loop fixed my issue