Typescript 枚举相等检查时编译错误

Question

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?

Answer 1

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");

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