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.

Beginne mit der verlinkten TypeScript Playground-Reproduktion und vergleiche die Diagnosen für getResult1 und getResult2. Verfolge, wie Nullish-Coalescing-Ketten und Ausdrücke mit Klammern die Typverengung beeinflussen, und füge einen Regressionstest hinzu, der beide äquivalenten Formen abdeckt. Erledigt ist die Aufgabe, wenn beide Formen value2.b als string inferieren und dasselbe akzeptierte Ergebnis liefern.