🔎 Search Terms

mapped type, narrow, optional, undefined, union

🕗 Version & Regression Information

This changed between versions 5.4 and 5.5. This can be reproduced in the playground.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.2#code/C4TwDgpgBAkgdhAHgQwMbAPJmASwPZzIA2AzgDwCCAfFALxQDeAUFFANoDSUOcUA1hBB4AZlApRkJKAFc4AEwjCeEOVCTAI8qRU4BdKAH4oXAFxQEANwgAnXQbOyFShKvWa52vS1aGxeqAA+MvKKynLerGY6HLoA3EwAvlAAZIzenNy8AkKi4pLBTmFqiBpafjG+ljZQZjFRevEJ8UygkLC89Myswnh49lAkwNY8AObxrABGyNZmcNIAthM241BTAF4OIc4qjUxMAPT7UACiiJDoKmYMUD19ZoPDcCOBBaEusavTswtL1h-r-Ucbx2UASLXA0Aw0mAdHaSDQmGw+EIpDI8CoTCAA

💻 Code

type InexactOptionals<A> = {
  [K in keyof A as undefined extends A[K] ? K : never]?: undefined extends A[K]
    ? A[K] | undefined
    : A[K];
} & {
  [K in keyof A as undefined extends A[K] ? never : K]: A[K];
};

type In = {
  foo?: string;
  bar: number;
  baz: undefined;
}

// Expected: { foo?: string | undefined; bar: number; baz?: undefined; }
type Out = InexactOptionals<In>

🙁 Actual behavior

The A[K] | undefined in the mapped type ternary is narrowed to A[K], meaning that foo in the example is foo?: string.

🙂 Expected behavior

The A[K] | undefined in the mapped type ternary is not narrowed, so we see foo?: string | undefined as expected.

Additional information about the issue

My use case is cheaply migrating a codebase to support exactOptionalPropertyTypes. It can also be useful for better React DX.