具有未知的Typecsript条件类型

type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

function sum(a: number, b: number): number {
  return a + b;
}

type SumReturnType = ReturnType<typeof sum>; // SumReturnType为number

type Person = {
  name: string;
  age?: number;
};

type AgeOrUnknown<T> = T extends { age: number } ? T['age'] : unknown;

const person1: Person = { name: 'Alice' };
const person2: Person = { name: 'Bob', age: 20 };

type Person1Age = AgeOrUnknown<typeof person1>; // Person1Age为unknown
type Person2Age = AgeOrUnknown<typeof person2>; // Person2Age为number

type ExtractString<T> = T extends string ? T : never;

type Values = 'a' | 1 | true;
type StringValues = ExtractString<Values>; // StringValues为'a'