[proposal] Non widened string values should be valid enum values, like widened string values

[daniele-orlando]

🔍 Search Terms

enum, string

✅ Viability Checklist

• 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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

TypeScript accepts widened strings as enum values, but refuses to accept non widened strings as enum values. I propose to relax this constraint allowing widened and non widened strings as valid enum values.

📃 Motivating Example

At the moment TypeScript accepts widened strings as valid enum values.

// FILE:./case1.ts
export const Case1_ImagePngType = 'image/png'

// Emitted in FILE:./case1.d.ts as
export declare const Case1_ImagePngType = "image/png"

// FILE:./test1.ts
import {Case1_ImagePngType} from './case1.js'

enum Case1 {
  // WORKS FINE
  Png = Case1_ImagePngType,
}

Trying to use a non widened string results in a compiler error.

// FILE:./case2.ts
export const Case2_ImageJpegType = 'image/jpeg' as const

// Emitted in FILE:./case2.d.ts as
export declare const Case2_ImageJpegType: "image/jpeg"

// FILE:./test2.ts
import {Case2_ImageJpegType} from './case2.js'

enum Case2 {
  // FAILS WITH: Type 'string' is not assignable to type 'number' as required for computed enum member values.
  Jpeg = Case2_ImageJpegType,
}

💻 Use Cases

Given a consumer project P1 using exported widened strings values from a consumed library L1, if L1 annotates the exported strings values as const, L1 breaks P1.

Given that a widened string value is a valid enum value, a "stricter" non widened string (a subset of the widened one) should be a valid enum value too.

// FILE:./lib-1.ts
export const ImagePngType = 'image/png'

// FILE:./lib-2.ts
export const ImageJpegType = 'image/jpeg' as const

// FILE:./project.ts
enum MyEnum {
  Png = ImagePngType, // FINE.
  Jpeg = ImageJpegType, // SHOULD BE FINE, BUT ERROR AT THE MOMENT.
}

[daniele-orlando]

Follows from #59187.

[RyanCavanaugh]

@ahejlsberg this seemed like a simple oversight to me but wasn't 100% sure. Can you weigh in?

[RyanCavanaugh]

Discussion notes:

• Allow interpreting const x: "foo" as a constant source for "foo"
• Allow seeing declare const x = "foo" as const in declaration files
  • Eventually change the declaration emitter to use that format instead