条件类型声明

type CheckProperty<T> = T extends { prop: infer P } ? P : never;

type Result = CheckProperty<{ prop: string }>;  // Result 类型为 string

type Filter<T, U> = T extends U ? T : never;

type Numbers = Filter<number | string, number>;  // Numbers 类型为 number

type Map<T> = { [K in keyof T]: T[K] extends string ? T[K] : never };

type Mapped = Map<{ a: string, b: number, c: boolean }>;
// Mapped 类型为 { a: string, b: never, c: never }

type OptionalToRequired<T> = {
  [K in keyof T]-?: T[K];
};

type RequiredProps = OptionalToRequired<{ a?: string, b?: number }>;
// RequiredProps 类型为 { a: string, b: number }