SOURCE

let money = 100
while (true){
let le = +prompt(`
    请您输入操作：
    1.存钱
    2.取钱
    3.查看余额
    4.退出  
   ` )
if (le === 4){
    break
}
switch(le) {
case 1:
let cun =  +prompt(`输入存款金额：`) 
money = cun + money
break
case 2:
let qu = + prompt (`输入取款金额：`)
money = money - qu 
if (money < 0){
    alert(`金额小于零，无法取出存款`)
break
}
case 3:
alert (`您的余额为：${money}`)
break
}
}