How to tell Typescript interpreter that variable will be a specific type in the runtime

I have a method that accepts argument of either string or MyEnum type and has a return type of only MyEnum. Also, there is a check – if a given argument is of MyEnum type, it will be returned immediately, otherwise additional logic is applied.

But when I want to do that I get this error:

Type ‘string | MyEnum’ is not assignable to type ‘MyEnum’.
Type ‘string’ is not assignable to type ‘MyEnum’.

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My code:

export enum MyEnum {
    Value1 = 1,
    Other = 10
}

export function TextToEnum(text: string | MyEnum): MyEnum {
    if (typeof text === typeof MyEnum)
        return text;

    switch (text) {
      case "val1":
        return MyEnum.Value1;
      default:
        return MyEnum.Other;
    }
}

>Solution :

Change your typeof check to use "number" instead of typeof MyEnum:

export function TextToEnum(text: string | MyEnum): MyEnum {
    if (typeof text === "number")
        return text;

    switch (text) {
      case "val1":
        return MyEnum.Value1;
      default:
        return MyEnum.Other;
    }
}

The reason why typeof MyEnum doesn’t work is because typeof always has the same string union type:

"string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"

and therefore conveys no type information.

Playground

Alternatively, you can check if it isn’t a string:

export function TextToEnum(text: string | MyEnum): MyEnum {
    if (typeof text !== "string")
        return text;

    // ...
}

and it’ll also work.

Playground