Type Error in Array.includes() method when the array contains only string keys

[LoganTann]

🔎 Search Terms

"typescript for in string" "typescript keyof for in" "array includes keyof"

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYGwhgzhAEAiYBcwGFxWgbwFDV9ADgK4BGIAlsNGAPwBc0ECATmQHYDmA3DnkaRdGJ1orQgFtiAUybcAvlmAB7Vo2hl2rRU0kATANKSAnjHoAKANZHFAMziIUaCAEoA2gF1oAXjwuA5GF83bgVlVR1gAEkVJFZgSS8RSQB3OyRUSAhTJ25wqMYwWMkAOjAEgCJlSTKcyOiCuKLiBIAmbmstaFMlaOhLQzVWaFy6wqdMHlwyW1MAQnVNbX0jCCK2UEIdSUy+pzHsPAPobohFEGKQRXZTMuH8wpcAUggPbyeygBpeo0+AZWY2K63GJxFx9Ny7bgHeTyLBAA

💻 Code

class DataClass {
    public a?: string;
    public b?: number;
}
const ignoredKeys : (keyof DataClass)[] =   ['a'];

const dcInstance = new DataClass();
dcInstance.a = "one";
dcInstance.b = 2;
for (const key in dcInstance) {
    if (!ignoredKeys.includes(key)) {
        console.log("dcInstance[%s] = %s", key, String(dcInstance[key]));
    }
}

🙁 Actual behavior

Actual behavior :

• In statement ignoredKeys.includes(key), error Argument of type 'string' is not assignable to parameter of type 'keyof DataClass'
• In statement dcInstance[key], error Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'DataClass'. No index signature with a parameter of type 'string' was found on type 'DataClass'.
• const key is typed as "string"

🙂 Expected behavior

The statement ignoredKeys.includes(key) should not throw any type errors. The Array.includes() method should accept strings as arguments when all items stored in the ignoredKeys array are string keys.

Other expected behavior, but not important at all :
- Variable key should be of type keyof typeof dcInstance instead of "string"
- No type errors in statement dcInstance[key]

Additional information about the issue

I have an entity and would like to iterate over the keys of that object, excluding some keys.

[MartinJohns]

Duplicate of #14520.

[nmain]

ignoredKeys.includes(key) should not throw any type errors

See #14520, and other duplicates linked from it. My search terms were "array includes lower bounds".

Variable key should be of type keyof typeof dcInstance instead of string

Types are not sealed, so this is not true in general. See #12936 for more details.

No type errors in statement dcInstance[key]

This follows from the above; once key is string, the indexing is no longer safe.

[LoganTann]

Linking another duplicate, thanks for the clarifications.

#55090

Closing this issue.

[LoganTann]

Except casting using unknown or any types, are there any workarounds for the simple example I provided here ?