Array.includes() should accept any arg that extends the array's type

[erikdstock]

🔎 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

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAtgTwIICcUEMEC4bRQSzAHMBtAXRgF4YSAidWgGhloCNayBYAKB6gQAOAUxgBZTKyEA5AK4AbOVVxQCxGAB8YYeXJ6hIsASiEQhYKADV0cmUOr1aMdBDETpOveGjx8ECISIrGxF7YEcnF3EESVkFTwMtHSDbJW0FCNdo9zjebn1vEgQTZjAQZmh8BSkQCmoSHhhG+GQ0TAA6QmAbABMTAAojEzNLa1sASkYGpsRUDAQOsC6ZXog+uF9-YmShCanGmdb5zp7+tLltsZ4yIA

💻 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

Relevant code.

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;}
// ...
}

[jcalz]

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 #14520

[erikdstock]

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 #14520

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]

#26255 (comment)