function calculate(){
    // 贷款金额
    var amount = document.getElementById('amount');
    // 年利息
    var apr = document.getElementById('apr');
    // 还款期
    var years = document.getElementById('years');
    //贷款方
    var zipcode = document.getElementById('zipcode');
    // 每月支付
    var payment = document.getElementById('payment');
    // 总支付
    var total = document.getElementById('total');
    // 总利息
    var totalinterest = document.getElementById('totalinterest');

    // amount 贷款总额
    var principle = parseFloat(amount.value);
    // apr 年利率
    var interest = parseFloat(apr.value) /100 /12;
    // 偿还周期 月份
    var payments = parseFloat(years.value) * 12;

    // pow 求次幂
    var x = Math.pow(1 + interest, payments);
    // monthly 每月还款金额
    var monthly = (principle * x * interest) / (x-1);

    //如果没有超过数字范围，且用户输入也正确
    if(isFinite(monthly)){
        payment.innerHTML = monthly.toFixed(2);
        total.innerHTML = (monthly * payments).toFixed(2);
        totalinterest.innerHTML = (monthly * payments - principle).toFixed(2);

        // 保存数据到本地
        save(amount.value, apr.value, years.value, zipcode.value);

        // 借贷人，忽略网络错误
        try{
            getLenders(amount.value, apr.value, years.value, zipcode.value);
        }
        catch (e) {
            //忽略异常
        }
        // 画图
        chart(principle, interest, monthly, payments);
    }else{
        //计算结果不是数字或者无穷大，意味着输入数据是非法或不完整的
        //清空数据
        payment.innerHTML = "";
        total.innerHTML = "";
        totalinterest.innerHTML = "";
        chart();
    }
}
//将用户输入保存至localSorage对象的属性中
function save(amount, apr, years, zipcode){
    if(window.localStorage){//只有支持的浏览器才能运行这个代码
        localStorage.loan_amount = amount;
        localStorage.loan_apr = apr;
        localStorage.loan_years = years;
        localStorage.loan_zipcode = zipcode;
    }
}
//文档首次加载的时，将会尝试还原输入字段
window.onload = function(){
    //如果浏览器支持本地存储并且上次保存的值也是存在的
    if(window.localStorage && localStorage.loan_amount){
        document.getElementById('amount').value = localStorage.loan_amount;
        document.getElementById('apr').value = localStorage.loan_apr;
        document.getElementById('years').value = localStorage.loan_years;
        document.getElementById('zipcode').value = localStorage.loan_zipcode;
    }
}

//将用户的输入发送至服务器上
function getLenders(amount,apr,years,zipcode) {
    //如果浏览器不支持XMLHttpRequest对象，则退出
    if(!window.XMLHttpRequest) return;

    //找到要显示放贷人列表的元素
    var ad =document.getElementById("lenders");
    if(!ad) return; //如果为空则退出

    //将用户的输入数据进行URL编码，并将查询参数附加在URL里
    var url ="getLenders.php" + //处理数据的URL地址
    "?amt="+