Too narrow typing for Array.prototype.includes

[kulak-at]

Bug Report

The type for Array.prototype.includes seems to be too narrow making it hard to use in many use cases. Currently Array<T> would use T as a parameter for searchElement which makes it impractical when used to check against optional types:

const arr = [
    "a",
    "b",
    "c",
];

const fn = (val: string | number): val is string => {
    return arr.includes(val);
}

The code above should work properly but instead val is too wide typing to be passed to it.

The issue is even more prevalent when used with as const keyword:

const arr = [
    "a",
    "b",
    "c"
] as const;

const isInConstArray = (val: string): boolean => {
    return arr.includes(val); // Error, Argument of type 'string' is not assignable to parameter of type '"a" | "b" | "c"'.
}

🔎 Search Terms

🕗 Version & Regression Information

Seems to be old bug, I could reproduce it in 3.9.7 which seems to be the first to understand .includes syntax on the Playground.

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

⏯ Playground Link

Playground link with relevant code

💻 Code

const arr = [
    "a",
    "b",
    "c"
];

const isInConstArray = (val: string | number): boolean => {
    return arr.includes(val);
}

🙁 Actual behavior

Error when passing val as parameter to arr.includes.

🙂 Expected behavior

arr.includes should accept wider type and behave like type guard instead.

[MartinJohns]

Duplicate of #26255 / #14520.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[whzx5byb]

Workaround:

interface ReadonlyArray<T> {
    includes<U>(this: T extends U ? this : never, searchElement: U): boolean;
}

const arr = [
    "a",
    "b",
    "c"
] as const;

declare var v1: string;
declare var v2: number;

arr.includes(v1); // OK
arr.includes(v2); // Error as expected

[amerllica]

Why has the @typescript-bot closed this issue? This issue is still being happened.

[MartinJohns]

@amerllica The bot has closed the issue because it is a duplicate of #26255 / #14520. Feel free to subscribe to the open issue #14520.