联合类型和类型保护

类型保护在需要联合类型的时候才会出现或需要类型保护(类型守护)。
先创建两个接口

interface Student{
    anjiao: boolean;
    say: () => {
    
    }
}
interface Teacher {
    anjiao: boolean;
    skill: () => {};
}

调用判断接口，所采用的类型守护方式(常用的有四种)
断言 as方式

function judgeWho(animal: Student| Teacher) {
    if(animal.anjiao){
        (animal as Teacher).skill()
    }else{
        (animal as Student).say()
    }
}

in 方式

function judgeWhoTow(animal: Student| Teacher) {
    if ('skill' in animal) {
        animal.skill();
    }else {
        animal.say();
    }
}

typeof方式

function add(first: string | number,second: string | number) {
    if(typeof first === 'string' || typeof second === 'string') {
        return `${first}${second}`;
    }
    return first + second;
}

instanceof方式

class NumberObj {
    count!: number; //非空用法
}
function addObj(first: object | NumberObj, second: object | NumberObj){
    if (first instanceof NumberObj && second instanceof NumberObj) {
        return first.count + second.count;
    }
    return 0;
}