Here's some code that I've been working on:
enum AnimalId {
Dog = 2,
Cat = 3
}
const animalIds: AnimalId[] = [AnimalId.Dog, 4];
I'm curious as to why TypeScript isn't flagging the assignment of 4 to type AnimalId[]. Shouldn't it be showing an error in this case?
The primary factor contributing to the variance in behavior stems from the capability to utilize flag number enums and employ bitwise operators with them. This functionality is not available for string enums.
Check out Daniel Rosenwasser's explanation on GH :
This approach is driven by the use of bitwise operations. There are instances where SomeFlag.Foo | SomeFlag.Bar is intended to yield another SomeFlag. However, you end up with a number instead, and the need to cast it back to SomeFlag can be cumbersome. In hindsight, if we were starting TypeScript anew and still utilizing enums, we would likely introduce a separate structure specifically designed for bit flags.
To address this issue, consider using strings in place of numbers:
enum AnimalId {
Dog = "2",
Cat = "3"
}
const animalIds: AnimalId[] = [AnimalId.Dog, 4]; // ❌ Type 'number' is not assignable to type 'AnimalId'.
Check out the TS Playground for more: https://www.typescriptlang.org/play?#code/KYOwrgtgBAgiCWECGAbAkgEygbwFBSgBEB7AcygF4oAiAJmoBp8oBhJAF0poGZrcBfXLgDGxEAGdOSBMnQZxALlgzUmANoBdLmriJVGAHQlSDKABYNAblxA
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