`Array.prototype.includes` should be narrow on union of enum types instead of throwing

[mtsoltan]

Suggestion

🔍 Search Terms

includes, enum (#46085 is relevant but not what this is discussing)

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Array.prototype.includes should be able to narrow down on a union of enum types instead of throwing:

TS2345: Argument of type 'SomeEnum | SomeOtherEnum' is not assignable to parameter of type 'SomeEnum'.   Type 'SomeOtherEnum.C' is not assignable to type 'SomeEnum'.

📃 Motivating Example

export enum SomeEnum {
    A = 'aaa',
    B = 'bbb'
}

export enum SomeOtherEnum {
    C = 'ccc',
    D = 'ddd'
}


  function someFunction(somePossibleValue: SomeEnum | SomeOtherEnum) {
    if (Object.values(SomeEnum).includes(somePossibleValue)) { // <--- Includes signature should be changed so this narrow the type at compile time
      return 'some result'; // <--- Inside the if-body, the type of somePossibleValue should be successfully narrowed down
    }
    if (Object.values(SomeOtherEnum).includes(somePossibleValue)) { // <--- Includes signature should be changed so this narrow the type at compile time
      return 'some other result'; // <--- Inside the if-body, the type of somePossibleValue should be successfully narrowed down
    }

    throw new Error('On neither'); // <--- I understand this cannot be figured out at compile time because .includes cannot execute at compile time
  }

💻 Use Cases

Allowing checks on something that could be possibly one of multiple enums.

[MartinJohns]

Duplicate of #26255 / #14520 and also #36275.

[mtsoltan]

🤦 I should've searched more before opening an issue. Apparently this has more to do with Array.prototype.includes in general, and not just with how it acts on enums. Feel free to close if you feel this would fork the discussion to pointless directions.

[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.