Typescript Union type with a function

As Titian already mentioned, you have to use parenthesis.

class TimePeriod {
    name: string | (() => string);
}

Another approach is to use a type alias:

type ATypeName = () => string;
class TimePeriod {
    name: string | ATypeName;
}

You just need an extra set of parenthesis.

class TestClass {
    name: string | (() => string);
}

The compiler is trying to do (string | ()) => string if you don't use them, because of precedence.

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