T extends U ? X : Y

type FormErrors<T> = {
  [K in keyof T]?: T[K] extends string 
    ? 'TOO_SHORT' | 'INVALID_FORMAT' 
    : T[K] extends number 
      ? 'OUT_OF_RANGE' | 'NAN_ERROR' 
      : 'FIELD_ERROR';
};

interface UserForm {
  name: string;
  age: number;
  preferences: string[];
}

// 自动推导出每个字段的错误类型
type UserFormErrors = FormErrors<UserForm>;
/*
{
  name?: "TOO_SHORT" | "INVALID_FORMAT";
  age?: "OUT_OF_RANGE" | "NAN_ERROR";
  preferences?: "FIELD_ERROR";
}
*/

type ApiResponse<T> = 
  T extends { paginated: true } 
    ? { data: T[]; total: number } 
    : T extends { list: true }
      ? T[]
      : T;

// 使用示例
type User = { id: number; name: string };

// 自动推导分页响应结构
type PaginatedUsers = ApiResponse<{ paginated: true; data: User }>;
// => { data: User[]; total: number }

// 自动推导列表响应结构
type UserList = ApiResponse<{ list: true; item: User }>;
// => User[]

type ExtractArrayType<T> = 
  T extends (infer U)[] ? U : never;

// 自动提取数组元素类型
type Numbers = ExtractArrayType<number[]>; // => number
type FirstItem = ExtractArrayType<[string, number]>; // => string

// 错误：条件类型直接用在变量上
type Wrong = number extends string ? true : false; // 永远为false

// 正确：通过泛型参数延迟求值
type Check<T> = T extends string ? true : false;
type Test = Check<number>; // => false