ts:条件类型

1. 条件类型表达式 T extends U ? X : Y

1. 基本用法--判断类型
// 判断是不是字符串
type IsString<T> = T extends string ? true : false;
type Result1 = IsString<string>;      // true  
//判断是不是数组
type isArray<T> = T extends Array<any> ? true : false
type testArray = isArray<'123'>
判断是不是对象
type isObject<T> T extends object > true : false
type testObject = isObject<{a:'123'}>
2. 提取元组的类型
type ArrayElementType<T extends any[]> = T extends (infer U)[] ? U : never;  
// 使用示例  
const numbers: number[] = [1, 2, 3];  
type NumberElementType = ArrayElementType<typeof numbers>; // number  
3. 排除null或undefined的类型
type noNull<T> = T extends null | undefined ? never : T
type Result1 = noNull<string | null>
4. 递归条件类型示例
type DeepArrayElementType<T> =  
  T extends any[]  
    ? T[number] extends infer U  
      ? U extends any[]  
        ? DeepArrayElementType<U>  
        : U  
      : never  
    : T;  
  
// 使用通用版本  
type Example5 = DeepArrayElementType<number[][][]>; // number  
type Example6 = DeepArrayElementType<string | number[][]>; // string | number
6. 条件烈类型与泛型类型结合使用

// 只针对接口的递归
//实现一个泛型 DeepReadonly<T>，它将对象的每个参数及其子对象递归地设为只读。
type X = {
  x: {
    a: 1;
    b: "hi";
  };
  y: "hey";
};

type Expected = {
  readonly x: {
    readonly a: 1;
    readonly b: "hi";
  };
  readonly y: "hey";
};

type DeepReadonly<T> = {
  readonly [P in keyof T]: T[P] extends object ? DeepReadonly<T[P]> : T[P];
};

let zs: DeepReadonly<X> = {
  x: {
    a: 1,
    b: "hi",
  },
  y: "hey",
};
zs.x.b = "xc";

infer

1. 提取数组元素类型
type ElementType<T extends any[]> = T extends (infer U)[] ? U : never;  
// 使用示例  
type NumArrayElementType = ElementType<[1, 2, 3, 4]>; // number  
type MixedArrayElementType = ElementType<[1, "two", true]>; // number | "two" | true，但通常这不是我们想要的
2. 提取数组最后一个元素类型
type arr1 = [1, 2, 3]
type FirstElement<T extends any[]> =  
  T extends [...infer First, infer Rest] ? Rest : never;  
type lastTest  = FirstElement<arr1>
3. 提取数组第一个元素类型
type arr2 = ['a', 'b', 'c']
type FirstElement<T extends any[]> =  
  T extends [infer First, ...infer Rest] ? First : never;  
type firstTest  = FirstElement<arr1>
4. 提取函数参数类型
type FunctionParams<T extends (...args: any) => any> = T extends (...args: infer P) => any ? P : never;  
type Params = FunctionParams<(a: string, b: number) => void>; 
5. 提取对象属性值类型
type ValueTypeOfProperty<T, K extends keyof T> = T[K] extends (infer V) ? V : never;  
type Obj = {  
  a: string;  
  b: number;  
};  
type AType = ValueTypeOfProperty<Obj, 'a'>;  
// AType 类型为 string   
6. 提取函数返回类型
type FunctionReturnType<T extends (...args: any) => any> = T extends (...args: any) => infer R ? R : never;  
type ReturnType = FunctionReturnType<() => string>;  
// ReturnType 类型为 string
7. 提取联合类型中的各个类型
8. 创建类型变换（如数组转换成元组）

2. Exclude

type T0 = Exclude<"a" | "b" | "c", "a" | "b">;  // "c"  
type T2 = Exclude<string | number | (() => void), Function>;  // string | number

3. Uppercase

type Greeting = "hello";  
type UppercaseGreeting = Uppercase<Greeting>; // 类型为 "HELLO"