Enum can be assigned out-of-bounds values

[jrieken]

TypeScript Version: 3.4.0-rc

const enum Fonum  {
    a = 1,
    b = 2
}

let value: Fonum = 12; // no error?
let value2: 1 | 2 = 12; // error

https://www.typescriptlang.org/play/#src=const%20enum%20Fonum%20%20%7B%0D%0A%20%20%20%20a%20%3D%201%2C%0D%0A%20%20%20%20b%20%3D%202%0D%0A%7D%0D%0A%0D%0Alet%20value%3A%20Fonum%20%3D%2012%3B%20%2F%2F%20no%20error%3F%0D%0A%0D%0Alet%20value2%3A%201%20%7C%202%20%3D%2012%3B%20%2F%2F%20error

[dragomirtitian]

I believe this is by design as stated by @DanielRosenwasser in #26362 to allow for bitwise operations on flag enums:

The behavior is motivated by bitwise operations. There are times when SomeFlag.Foo | SomeFlag.Bar is intended to produce another SomeFlag. Instead you end up with number, and you don't want to have to cast back to SomeFlag.
I think if we did TypeScript over again and still had enums, we'd have made a separate construct for bit flags.

[jrieken]

bitwise operations on flag enums:

That makes sense but then they should geek out and understand that you cannot produce 12 with 1 and/or 2

[dragomirtitian]

@jrieken Challenge accepted 😄 :

let value: Fonum = Fonum.a << Fonum.b << Fonum.a  |  Fonum.a << Fonum.b; // 12

I think it is difficult to tell if the resulting number is valid or not if we consider flag enums and the whole range of bitwise operations.

[jrieken]

haha - fair point. There should probably be strict enum that doesn't allow these things...