num = prompt('操作:\n1.存钱\n2.取钱\n3.显示余额\n4.退出');这句话的作用是什么

var xiancun = 100;
        
        var num = prompt('请输入您的操作：\n 1.存钱\n 2.取钱\n 3.显示余额\n 4.退出');
        while(xiancun>=10){
            if (num == '1') {
            var jia = prompt('请您输入存的钱数');
            xiancun = xiancun + parseInt(jia);
            alert('您现在的钱数是:' + xiancun);
            num = prompt('操作:\n1.存钱\n2.取钱\n3.显示余额\n4.退出');
            // alert(prompt('操作:\n1.存钱\n2.取钱\n3.显示余额\n4.退出'));
        } else if (num == '2') {
            var jia = prompt('请您输入取的钱数');
            xiancun = xiancun - parseInt(jia);
            alert('您现在的钱数是:' + xiancun);
            num = prompt('操作:\n1.存钱\n2.取钱\n3.显示余额\n4.退出');

        } else if (num == '3') {
            alert('您现在的钱数是:' + xiancun);
            num = prompt('操作:\n1.存钱\n2.取钱\n3.显示余额\n4.退出');

        } else if (num == '4') {
            num=false;
            break;
        }else{
            num = prompt('操作:\n1.存钱\n2.取钱\n3.显示余额\n4.退出');

        }
        }

提示并等待输入