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Enums as union types aren't compatible with strings (5.0) #53304
Description
Bug Report
In https://devblogs.microsoft.com/typescript/announcing-typescript-5-0/#all-enums-are-union-enums, it mentions that enums can be used as union types, allowing to do this:
The issue is that this seems to only work with numbers, but not with other types like strings. If we take the exact same code but replace all numbers with strings, we have this:
Type '"42"' is not assignable to type 'E'. (2322)
🔎 Search Terms
enum, union, string
🕗 Version & Regression Information
This new feature was introduced in the v5.0.0, and I tested it locally in the v5.0.2 and in the sandbox with the version 5.1.0-dev.20230316.
⏯ Playground Link
Playground link with relevant code
💻 Code
enum E {
A = '42',
}
export let fine: E = '42'; // fails but should work
// ^
export let oops: E = '43'; // fails as expected
// ====
enum F {
Foo = '10',
Bar = '20',
}
function takeValue(e: F) {}
takeValue(F.Foo); // works
takeValue('10'); // fails but should work
// ^
takeValue('123'); // fails as expected🙁 Actual behavior
The error Type '"42"' is not assignable to type 'E'. (2322) is sent by TS for the code let fine: E = '42';.
🙂 Expected behavior
This shouldn’t behave differently from numbers, so writing let fine: E = '42'; could be allowed by TS
Activity
[-]Enums as union types aren't compatible with strings[/-][+]Enums as union types aren't compatible with strings (5.0)[/+]fatcerberus commentedon Mar 17, 2023
AFAIK string enums are intentionally stricter than numeric ones, so this is probably by design.
notaphplover commentedon Mar 17, 2023
It's doesn't seem to be a bug. Consider this message from the lead architect of TypeScript.
Ayc0 commentedon Mar 17, 2023
Thanks for the information! 🙏
If that's expected, should the release note be updated to highlight the fact that this only works for numbers and not for all types?
It mentions that enums started as "a set of numeric constants with the same type" and that in the example "E.Foo and E.Bar (...) were pretty much just numbers".
But it doesn’t explicitly say that the union feature can only work with numbers (specially when it mentions "Enum literal types gave each enum member its own type", and later "That means that all enums can now be narrowed and have their members referenced as types as well", which can lead to a bit of confusion).
Ayc0 commentedon Mar 17, 2023
I think you’re right notaphplover, this comment was a reply of a question in the PR exactly about the behavior mentioned by this issue. So it doesn’t seem to be a bug.
I’ll close this as completed.
devlopersabbir commentedon Jul 29, 2024
Here is a simple way to do that.
Here the example of how to convert a Typescript enum to a union type
And it’s pretty easy
Reference Link