function C(amount){

this.balance=amount;

this.deposit=deposit;

this.withdraw=withdraw;

this.toString=toString;

}

function deposit(amount){

this.balance+=amount;

}

function withdraw(amount){

if(amount<=this.balance){

this.balance-=amount;

}

if(amount>this.balance){

alert("余额不足");

}

}

function toString(amount){

return "余额:"+this.balance;

}

var account=new C(500);

account.deposit(1000);

console.log(account.toString());

account.withdraw(750);

console.log(account.toString());

account.withdraw(375);

console.log(account.toString());