TypeScript compiler error for Enum in Switch statement

Question

Why does the TypeScript compilier (version 2.7.2) show the error message "error TS2678: Type 'Enum1.b' is not comparable to type 'Enum1.a'" for the following code sequence?

const enum Enum1 {a, b};

let e1: Enum1 = Enum1.a;

for (let i = 0; i < 10; i++) {
   const newE1 = (i % 2 == 0) ? Enum1.a : Enum1.b;
   setE1(newE1);
   switch (e1) {
      case Enum1.a:
         console.log("a");
         break;
      case Enum1.b:
         console.log("b");
         break;
   }
}

function setE1 (newE1: Enum1) {
   e1 = newE1;
}

When I replace switch(e1) with switch(+e1), the error does not occur.

basarat · Accepted Answer · 2018-02-22 04:05:36Z

1

Simplification :

const enum Enum1 {a, b};
let e1: Enum1 = Enum1.a;

setE1(Enum1.b);
// Error === cannot be applied to Enum1.a and Enum1.b
if (e1 === Enum1.b) console.log('yup'); // But it is

function setE1(newE1: Enum1) {
   e1 = newE1;
}

Seems like a bug in the inference engine. You should report it here : https://github.com/Microsoft/TypeScript/issues

answered Feb 22, 2018 at 4:05

basarat

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3 Comments

Christian d'Heureuse Over a year ago

Thanks, I have reported it here

Christian d'Heureuse Over a year ago

I have found another workaround: let e1: number = Enum1.a; instead of let e1: Enum1 = Enum1.a;

Christian d'Heureuse Over a year ago

Another workaround: let e1: Enum1 = <Enum1>Enum1.a; or let e1 = <Enum1>Enum1.a;

user1067920 · Accepted Answer · 2020-01-15 10:22:45Z

0

If you use the switch expression in a function with typed parameter, this works as expected.

Example:

const enum Enum1 {a, b};

function testSwitch(e1: Enum1) {
    switch (e1) {
        case Enum1.a:
            console.log("a");
            break;
        case Enum1.b:
            console.log("b");
            break;
    }
}

testSwitch(Enum1.a)

will compile 🥳 and output a 👍

answered Jan 15, 2020 at 10:22

user1067920

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