Comparability of Enum values

[m1gu3l]

TypeScript Version: 2.0.2 Release Candidate

Code

enum Place {
    Bottom,
    Left,
    Right,
    Top
}

let p: Place = Place.Left;
console.log(p === Place.Left);
console.log(p === Place.Right);

Expected behavior:
true
false

Actual behavior:
error TS2365: Operator '===' cannot be applied to types 'Place.Left' and 'Place.Right'.

[m1gu3l]

#10119 and #10120 might be related

[ahejlsberg]

This is working as intended. The compiler performs control flow analysis and knows that p has the actual value Place.Left where you're performing the === operations, and it is calling out that it makes no sense to compare a value that is known to be Place.Left to a value that is known to be Place.Right. It's effectively like writing Place.Left === Place.Right or 1 === 2, both of which would also produce errors.

[m1gu3l]

Oh, thanks for clarifying that. It still feels a bit weird.

It's effectively like writing Place.Left === Place.Right or 1 === 2, both of which would also produce errors.

Well no, it's like writing let p = 0 and then checking p === 1, which would produce false.

[ahejlsberg]

The thing that triggers the stricter behavior is that the type of the variable is one with a finite set of choices (i.e. a union type or an enum type). For example, the following is also an error

let p: 0 | 1 = 0;
console.log(p === 1);  // Error, comparing 0 to 1

[m1gu3l]

I understand.
It seems to be an overkill though. Sometimes developer might want to do check like that. And what about generated code?

[RyanCavanaugh]

You can write let p: Place = <Place>Place.Left; if you want to "erase" the known value of the variable.

In reality this check actually found a lot of bugs when it was created. People would check for an enum value and return, then check for it again (creating an unreachable block), etc.

Generated code should never be in a bad state due to this since the generator itself can know when it would emit unreachable blocks. And usually you wouldn't typecheck generated code anyway (since it ought to be correct).

[m1gu3l]

In ideal world there would be a flag to disable this globally, but I've noted my position is in the minority.