ts中的联合类型和类型保护

interface Bird{
    
    fly:boolean;
    sing:()=>{
    };
}
interface Dog{
    
    fly:boolean;
    bark:()=>{
    };
}
// 1.类型断言的方式做类型保护
function trainAnimal(animal:Bird|Dog){
    
    if(animal.fly){
    
        (animal as Bird).sing();
    }else{
    
        (animal as Dog).bark();
    }
}

// 2.in 语法来做类型保护
function trainAnimalSecond(animal:Bird|Dog){
    
    if('sing' in animal){
    
        animal.sing();
    }else{
    
        animal.bark();
    }
}

//3.typeof 语法做类型保护
function add(first :string | number,second:string | number){
    
    if(typeof first ==='string' || typeof second==='string'){
    
        return `${
      first}${
      second}`;
    }
    return first+second;
    
}

// 4.使用instanceof 语法来做类型保护，下面用到了instanceof 所有只能用class，不能用///interface定义接口
class NumberObj{
    
count:number
}
function addSecond(first:object | NumberObj,second: object | NumberObj){
    
    if(first instanceof NumberObj && second instanceof NumberObj){
    
        return first.count+second.count;
    };
    return 0;
}