Suggestion: Infer literal types for properties of "frozen" object

[levenleven]

Search Terms

Object.freeze, literal types

Suggestion

Infer literal types for properties of "frozen" object. See example below

Use Cases

Moving project from flow to typescript. Common pattern to define enums in flow is freezing an object and using union of a value types as the enum type:

const foo = Object.freeze({ a: 'a', b: 'b' });
type FooValue = $Values<typeof foo>; // --> 'a' | 'b'

Currently same pattern in typescript will result in string.

Examples

const foo = Object.freeze({ a: 'a', b: 'b' });
type Foo = typeof foo; // Expected: { readonly a: 'a'; readonly b: 'b'; } Actual: { readonly a: string; readonly b: string; }
type FooValue = Foo[keyof Foo] // Expected: 'a' | 'b' Actual: 'string'

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code (Not sure about that)
• 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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[fatcerberus]

Do note that TypeScript has native strongly-typed enums built in:
https://www.typescriptlang.org/docs/handbook/enums.html#main-content

Something like this might be nice to have when typechecking JS code (--checkJs) though.

[levenleven]

Sure, enums are just example. Once object frozen, its values can't be changed, so it makes sense to infer their types as lietrals

[NN---]

I guess this one will solve it once merged #29510

[levenleven]

@RyanCavanaugh just to clarify duplicate of #29510?

[typescript-bot]

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