Typescript Union type with a function

Question

I'm trying to have a property with a union type of a lambda function or a string.

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

Non-working TS playground sample can be accessed here.

But the TS compiler gives an error: "[ts] Member 'string' implicitly has an 'any' type."

Is the type declared incorrectly? Or is there a workaround for this?

Titian Cernicova-Dragomir · Accepted Answer · 2017-12-11 08:26:36Z

151

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.

answered Dec 11, 2017 at 8:26

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Comments

str · Accepted Answer · 2017-12-11 08:29:26Z

17

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;
}

answered Dec 11, 2017 at 8:29

str

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