CODEWITHSUNDEEP
typescript
joewalker
joewalker

Reputation: 35

Union types fail as a generic type parameter

The following code looks correct to me, however TypeScript reports an error:

type AorB = 'a' | 'b';

interface Container {
    aorb: AorB,
};

function example<T>(t: T): T {
    return t;
}

const aorb: AorB = example({ aorb: 'a' });
/*
Type '{ aorb: string; }' is not assignable to type 'AorB'.
  Type '{ aorb: string; }' is not assignable to type '"b"'.
*/

It looks as though the captured type is { aorb: string } rather than { aorb: AorB }. What's the best way to prevent this?

See it in the TypeScript playground

Upvotes: 0

Views: 71

Answers (1)

Karol Majewski
Karol Majewski

Reputation: 25790

Your AorB type is a string, but you passed an object to example. It should be either:

const aorb: AorB = example('a');

or

const container: Container = example({ aorb: 'a' as 'a' });

Upvotes: 3

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