I created this typescript file to show the problem:
enum E {
one,
two
}
console.log(E.one)
console.log(E.two)
let n: E = E.one
if (n === E.one) console.log("equal");
if (n === E.two) console.log("equal");
I get compile error from tsc on second if:
test.ts:11:5 - error TS2367: This condition will always return 'false' since the types 'E.one' and 'E.two' have no overlap.
I can't find out why it is happening. Am I doing something wrong?
This is intended behaviour (see this issue).
TypeScript's control flow analysis is smart enough to see that you just assigned E.one to n. There is now "invisible" type information associated with n which narrows down its type to E.one and will lead to the error in the if-statement because it knows that the condition will always evaluate to false.
If you want to avoid this behaviour, you can use the as operator which lets TypeScript conveniently forget the narrowed type information about n.
let n: E = E.one as E
if (n === E.one) console.log("equal");
if (n === E.two) console.log("equal");
