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Array.includes() should accept any arg that extends the array's type #60073
Description
🔎 Search Terms
array.includes, types
🕗 Version & Regression Information
- This is the behavior in every version I tried - the type definition is 7 years old.
⏯ Playground Link
💻 Code
No response
🙁 Actual behavior
I should not receive type errors because an argument of type string | null might be included in an array of type string[]. It is the job of the method to make the final determination.
🙂 Expected behavior
I get type errors because Array<string>.includes() will only accept a string value. I have to cast my argument to a string in order to perform the check.
Additional information about the issue
Basic suggested change:
interface Array<T> {
/**
* Determines whether an array includes a certain element, returning true or false as appropriate.
* @param searchElement The element to search for.
* @param fromIndex The position in this array at which to begin searching for searchElement.
*/
- includes(searchElement: T, fromIndex?: number): boolean;
+ includes<U extends T>(searchElement: U, fromIndex?: number): boolean;}
interface ReadonlyArray<T> {
/**
* Determines whether an array includes a certain element, returning true or false as appropriate.
* @param searchElement The element to search for.
* @param fromIndex The position in this array at which to begin searching for searchElement.
*/
- includes(searchElement: T, fromIndex?: number): boolean;
+ includes<U extends T>(searchElement: U, fromIndex?: number): boolean;}
// ...
}
Activity
jcalz commentedon Sep 26, 2024
That change wouldn’t help; it already accepts subtypes of the array element. What you want is the opposite (since
string extends (string | null)and not vice versa). This is a duplicate of #26255 . Ultimately you’re looking for #14520erikdstock commentedon Sep 26, 2024
Thanks for the link. I see there is a lot of discussion here - I searched but apparently missed it. In use case it seemed wrong to have to cast in this relatively straightforward
["a", "b"].includes(stringOrNull as string). If this has been decided years ago and I'm not contributing anything new feel free to close.WIP PR was here (first time contributing so I may have been doing it wrong).
RyanCavanaugh commentedon Sep 26, 2024
#26255 (comment)