typescript 的几种基本的类型保护

interface Brid {
    fly: boolean
    sing: () => {}
}

interface Dog {
    fly: boolean
    bark: () => {}
}

// 联合类型仅提示共有属性
function tranAnial(animal: Brid | Dog) {
    // 类型断言 实现类型保护
    if (animal.fly) {
        (animal as Brid).sing()
    } else {
        (animal as Dog).bark()
    }
}

function tranAnialSecond(animal: Brid | Dog) {
    // in 语法实现类型保护
    if ('sing' in animal) {
        animal.sing()
    } else {
        animal.bark()
    }

}

function add(first: string | number, second: string | number) {
    // typeof 语法实现类型保护
    if (typeof first === 'string' || typeof second === 'string') {
        return `${first}${second}`
    }
    return first + second
}

// 使用 instanceof 语法做类型保护
class NumberObj {
    // 声明变量
    count: number
    // 创建构造函数
    constructor(count: number) {
        this.count = count
    }
}
function addSecond(first: object | NumberObj, second: object | NumberObj) {
    if (first instanceof NumberObj && second instanceof NumberObj) {
        return first.count + second.count
    }
    return 0
}