Initializing enum members with another string enum causes incorrect/inconsistent types

[devrelm]

TypeScript Version: 2.4.1 through 3.1.0-dev.20180817

Search Terms:

• string enum values act like numbers
• inconsistent string enum types
• string enum of string enum

Code

enum Foo {
    Foo1 = 'hello',
    Foo2 = 'goodbye',
}

enum Bar {
    Bar1 = Foo.Foo1,
    Bar2 = Foo.Foo2,
}

const myStringFunc = (baz: string) => console.log(baz);

const myFooFunc = (baz: Foo) => myStringFunc(baz);

const myBarFunc = (baz: Bar) => myStringFunc(baz);
                                          // ~~~ Argument of type 'Bar' is not
                                          // assignable to parameter of type 'string'.

// This compiles fine! TypeScript thinks baz is a number!
const myOddFunc = (baz: Bar) => baz.toExponential();

Expected behavior:
String enums like Bar that are initialized with other string enum values should be properly typed as a string enum.

Actual behavior:
Bar is inconsistently typed as a numeric enum, as evidenced by baz.toExponential() not causing a compiler error.

To make it more fun, the members of Bar even autocomplete and hint as the correct string literals:

But the members of Bar themselves autocomplete members of Number.prototype:

Playground Link:
here

Related Issues:
None that I could find

[RyanCavanaugh]

The declaration of Bar should be an error, though I'm pretty afraid to change it at this point

[AnyhowStep]

When I have to do something like getting the subset of an enum, I just do,

enum E {
    A = "Hi",
    B = "Bar",
}
const SubEnum : {
    "A" : E.A,
    "B" : E.B
} = {
    A : E.A,
    B : E.B
}

With some small effort, you can write a function that creates these sub enums. But it would be nice if what you tried worked correctly, as it's intuitive.

[trusktr]

@AnyhowStep But then you can't use SubEnum as a type the same way as E, for example

function foo(arg: SubEnum) {}

won't work the same way as

function foo(arg: E) {}

Because the first one will expect that you pass in an object containing two properties, while the second one expects that you pass an arg which is one of the things inside the enum.

How do we make a subtype of an enum, so that we can use it like an enum, and its values are assignable to the superset enum, and we can use it like an enum type?

[s-h-a-d-o-w]

@trusktr I suppose that's a related but still quite different issue than what has been reported here. But I currently have problems with that as well (need to construct subsets of one big string enum that can even overlap at times), so if you've found a solution in the meantime, please let me know.

Update: Just in case someone else stumbles across this - I suppose since enums are intentionally "dumb", something this complex is unlikely to ever make it into the language.
And so I've instead resolved this by simulating the behavior using string literal types and wrote a detailed article about it.

[jakebailey]

Old issue, but this was fixed back in TS 5.0 with all of the new enum behavior updates.