TypeScript 类型学习3

/**
 * 操作符in 来进行类型缩小
 * js中in运算符用来确定对象是否具有某个名称的属性
 * 所以ts中也有这个，用来缩小潜在的类型范围
 */

//例如“value” in x ,value是字符串文字 x代表联合类型
//true x具有可选或者必需属性的类型的值
//false 分支的缩小 x需要具有可选或者缺失属性类型的值
type WJH = { name: string; age: number };
type WH = { name: string; color: string };
function Earth(people: WJH | WH) {
  if ("age" in people) {
    console.log(people.name);
  }
  if ("color" in people) {
    console.log(people.name);
  }
}
Earth({
  age: 18,
  name: "王佳慧",
});
Earth({
  color: "blue",
  name: "呵呵呵呵呵",
});

/**
 * instanceof 操作符缩小
 * 测试它左边的对象是否是它右边的类的实例，返回 boolean 的数据类型。
 * x instanceof A
 */
function logValue(x: Date | string) {
  if (x instanceof Date) {
    console.log(x.toUTCString());
  } else {
    console.log(x.toUpperCase());
  }
}
logValue(new Date());
logValue("wangjiahui");

/**
 * 类型谓词
 * 想要直接控制整个代码的类型变化
 * 想要定义一种类型的类型保护
 * 我们只需要定一个函数让他的返回值是类型谓词就可以
 */
type Fish = { swim: () => void; name: string };
type Bird = { fly: () => void; name: string };
function isFish(anmial: Fish | Bird): anmial is Fish {
  console.log((anmial as Fish).swim !== undefined);
  return (anmial as Fish).swim !== undefined;
}
//
function checkType<T>(val: any, type: keyof T): val is T {
  return (val as T)[type] !== undefined;
}
function getAnmialType(): Fish | Bird {
  let fish: Fish = {
    name: "鲨鱼",
    swim: () => {
      console.log("我是鲨鱼呵呵呵");
    },
  };
  let bird: Bird = {
    name: "麻雀",
    fly: () => {
      console.log("我是麻雀嘻嘻嘻");
    },
  };
  return true ? fish : bird;
}
let pet = getAnmialType();

if (isFish(pet)) {
  pet.swim();
} else {
  pet.fly();
}

//类型兼容  要满足之前的基础属性  TS的反推
//对象的赋值
interface A {
  name: string;
  age: number;
}
let B: A;
let C = {
  name: "wangjiahui",
  age: 10,
  color: "blue",
};
B = C;