const f = (_a: string, _b: string): void => {}; interface A { a?: string; b?: string; } interface B { a: string; b: string; } type U = A | B; const u: U = {} as any; // u.b narrowed, but not a // Argument of type 'string | undefined' is not assignable to parameter of type 'string'. // Type 'undefined' is not assignable to type 'string'. u.a && u.b && f(u.a, u.b); // Error: u.a narrowed, but not b // Argument of type 'string | undefined' is not assignable to parameter of type 'string'. // Type 'undefined' is not assignable to type 'string'. u.b && u.a && f(u.a, u.b); // Works typeof u.a === "string" && typeof u.b === "string" && f(u.a, u.b); // Works const a: A = {} as any; a.b && a.a && f(a.a, a.b);