Parentheses in nullish coalescing chains impacts type narrowing

Bug Report

🔎 Search Terms

nullish coalescing chain parentheses type narrowing prettier

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about nullish coalescing. Note that at the time of writing, 4.3.5 was the latest stable release.

⏯ Playground Link

Playground link with relevant code

💻 Code

type T =
  | { a: string; b: string }
  | { a: string; b?: undefined }
  | { a?: undefined; b: string };

const getResult1 = (value1: string | undefined, value2: T): string => {
  return value1 ?? value2.a ?? value2.b;
};

const getResult2 = (value1: string | undefined, value2: T): string => {
  return value1 ?? (value2.a ?? value2.b);
};

🙁 Actual behavior

• getResult1 throws a type error, as TypeScript incorrectly infers that value2.b has a type of string | undefined.
• getResult2 does not throw a type error, as TypeScript correctly infers that value2.b has a type of string.

🙂 Expected behavior

The existence and placement of parentheses in nullish coalescing chains should not impact type narrowing, as to my knowledge (and prettier, as it automatically removes the parentheses seen in getResult2), x ?? y ?? z and x ?? (y ?? z) actually behave the same at runtime.

Commencez par la reproduction liée dans TypeScript Playground et comparez les diagnostics de getResult1 et getResult2. Suivez l’effet des chaînes de nullish coalescing et des expressions parenthésées sur le narrowing des types, puis ajoutez un test de régression couvrant les deux formes équivalentes. C’est terminé lorsque les deux formes infèrent value2.b comme étant de type string et produisent le même résultat accepté.