function 函数名(形参：参数类型【参数类型大多数为any】): 形参 is A 类型{

    return true or false

}

class Customer {
    pay(payMethod: BankPay | MobilePay) {
        if (isMobilePay(payMethod)) {
            payMethod // MobilePay
        }
    }
}

// 自定义守卫
function isMobilePay(payMethod: any): payMethod is MobilePay {
    return payMethod instanceof MobilePay
}

function cal(num: string | number) {
    // num = num + 10 // 错误
    if (isNum(num)) {
        num + 3
    } else {
        num + 10
    }
}

function isNum(num: any): num is number {
    return typeof num === 'number'
}

export { }