联合类型和类型保护

interface Bird{
    fly:boolean;
    sing:()=>{}
}
interface Dog{
    fly:boolean;
    bark:()=>{}
}
//不会提示sing和bark方法需要判断，直接使用会报错
//两种常用方法:一种用 as 一种用 in
function trainAnial(animal:Bird|Dog){
    if(animal.fly){
        (animal as Bird).sing()
    }else{
        (animal as Dog).bark()
    }
}
function fn1(animal:Bird|Dog){
    if('sing' in animal){
        animal.sing()
    }else{
        animal.bark()
    }
}
//一个为字符串就会报错
function add (first:number|string,secound:number|number){
    if(typeof first=='string'||typeof secound=='string'){
        return 0
    }
    return first+secound
}
//必须为类
class NumberObj{
    count:number
}
function addA(first:object|NumberObj,secound:object|NumberObj){
    if(first instanceof NumberObj&&secound instanceof NumberObj){
    return first.count+secound.count

    }
    return 0
}