今天花了两个小时模仿书上代码用JS制作了JavaScript贷款计算器，时间有些长，但相比以前，自己细心了不少，每天进步一点点，量的积累达到质的飞跃
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<style type="text/css">
.output{
font-weight:bold;}
#payment{
text-decoration:underline;}
#graph{
border:1px solid black;}
th,td{
vertical-align:top;}
</style>
<script type="text/javascript">
function calculate(){
//查找文档中用于输入输出的元素
var amount=document.getElementById("amount");
var apr=document.getElementById("apr");
var years=document.getElementById("years");
var zipcode=document.getElementById("zipcode");
var payment=document.getElementById("payment");
var total=document.getElementById("total");
var totalinterest=document.getElementById("totalinterest");
//假设输入合法将从input元素中获取输入数据
//将百分比格式转换为小数格式，且从年利率转换为月利率
//将年度赔付转换为月度赔付
var principle=parseFloat(amount.value);
var interest=parseFloat(apr.value)/100/12;
var payments=parseFloat(years.value)*12;
var x=Math.pow(1+interest,payments);
var monthly=(principle * x * interest)/(x-1);
//如果结果没有超过javascript能表示的数字范围，且用户的输入也正确
//这里所展示的结果就是合法的
if(isFinite(monthly)){
//将数据填充至输出字段的位置，四舍五入到小数点后两位数字
payment.innerHTML=monthly.toFixed(2);
total.innerHTML=(monthly * payments).toFixed(2);
totalinterest.innerHTML=((monthly * payments)-principle).toFixed(2);
//将用户数据保存下来，以便在下次访问时也能取到数据
save(amount.value,apr.value,years.value,zipcode.value);
try{
getLenders(amount.value,apr.value,years.value,zipcode.value);
}catch(e){}
//用图表展示贷款余额，利息和资产收益
chart(principle,interest,monthly,payments);
}
else{
//计算结果不是数字或者无穷大，意味着输入的数据是非法的或是不完整的
//清空之前的输出数据
payments.innerHTML="";//清空元素的文本内容
total.innerHTML=""
totalinterest.innerHTML="";
chart();//不传参数就清空图表
}
}
function save(amount,apr,years,zipcode){
if(window.localStorage){
localStorage.loan_amount=amount;
localStorage.loan_apr=apr;
localStorage.loan_years=years;
localStorage.loan_zipcode=zipcode;
}
}
window.onload=function(){
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;
}
};
function getLenders(amount,apr,years,zipcode){
if(!window.XMLHttpRequest) return;
var ad=document.getElementById("lenders");
if(!ad) return;
var url="getLenders.php"+"?amt=" +encodeURIComponent(amount)+"&apr="+encodeURIComponent(apr)+"&yrs="+encodeURIComponent(years)+"&zip="+encodeURIComponent(zipcode);
//通过XMLHttpRequest对象来提取返回数据
var req=new XMLHttpRequest();
req.open("GET",url);
req.send(null);
req.onreadystatechange=function(){
if(req.readyState==4 && req.status==200){
var response=req.responseText;
var lenders=JSON.parse(response);
var list="";
for(var i=0;i<lenders.length;i++){
list +="<li><a href=‘" + lenders[i].url +"‘>" +lenders[i].name +"</a>";
}
ad.innerHTML="<ul>" + list + "</ul>";
}
}
}
function chart(principle,interest,monthly,payments){
var graph=document.getElementById("graph");
graph.width=graph.width;//清除且重置画布
//若不传入参数，或浏览器不返回，则直接返回
if(arguments.length==0 || !graph.getContext) return;
var g=graph.getContext("2d");
var width=graph.width,
height=graph.height;
function paymentToX(n){
return n * width/payments;
}
function amountToY(a){
return height-(a*height/(monthly * payments *1.05));
}
g.moveTo(paymentToX(0),amountToY(0));
g.lineTo(paymentToX(payments),amountToY(payments * monthly));
g.lineTo(paymentToX(payments),amountToY(0));
g.closePath();
g.fillStyle="#f88";
g.fill();
g.font="12 bold sans-serif";
g.fillText("Total Interest Payment",20,20);
var equity=0;
g.beginPath();
g.moveTo(paymentToX(0),amountToY(0));
for(var p=1;p<=payments;p++){
var thisMonthsInterest=(principle-equity) * interest;
equity +=(monthly-thisMonthsInterest);
g.lineTo(paymentToX(p),amountToY(equity));
}
g.lineTo(paymentToX(payments),amountToY(0));
g.closePath();
g.fillStyle="green";
g.fill();
g.fillText("Total Equity",20,35);
var bal=principle;
g.beginPath();
g.moveTo(paymentToX(0),amountToY(bal));
for(var p=1;p<=payments;i++){
var thisMonthsInterest=bal * interest;
bal -=(monthly-thisMonthsInterest);
g.lineTo(paymentToX(p),amountToY(bal));
}
g.lineWidth=3;
g.stroke();
g.fillStyle="black";
g.fillText("Loan Balance",20,50);
g.textAlign="center";
var y=amountToY(0);
for(var year=1;year * 12 <=payments;year++){
var x=paymentToX(year * 12);
g.fillRect(x-0.5,y-3,1,3);
if(year==1) g.fillText("Year",x,y-5);
if(year % 5 ==0 && year * 12 !==payments)
g.fillText(Sring(year),x,y-5);
}
//将赔付的金额标记在右边界
g.textAlign="right";//文字右对齐
g.textBaseline="middle";//文字垂直对其
var ticks=[monthly * payments,principle];//所用的两个点
var rightEdge=paymentToX(payments);//设置X坐标
for(var i=0;i<ticks.length;i++){//对每两个大做循环
var y=amountToY(ticks[i]);//计算每个标记的Y轴坐标
g.fillRect(rightEdge-3,y-0.5,3,1);//绘制标记
g.fillText(String(ticks[i].toFixed(0)),rightEdge-5,y);//绘制文本
}
}
</script>
</head>
<body>
<table>
<tr>
<th>Enter Loan Data:</th>
<td></td>
<th>Loan Balance,Cumulative Equity,and Interest Payments</th>
</tr>
<tr>
<td>Amount of the Loan($):</td>
<td><input id="amount" onChange="calculate();"></td>
<td rowspan="8">
<canvas id="graph" width="400" height="250"></canvas>
</td>
</tr>
<tr>
<td>Annual interest(%):</td>
<td><input id="apr" onChange="calculate();"></td>
</tr>
<tr>
<td>Repayment period(years):</td>
<td><input id="years" onChange="calculate();"></td>
<tr><td>Zipcode(to find lenders):</td>
<td><input id="zipcode" onChange="calculate();"></td>
<tr><th>Approximate Payments:</th>
<td><button onClick="calculate();">Calculate</button></td>
</tr>
<tr>
<td>Monthly payment:</td>
<td>$<span class="output" id="payment"></span></td>
</tr>
<tr>
<td>Total payment:</td>
<td>$<span class="output" id="total"></span></td>
</tr>
<tr>
<td>Total interest:</td>
<td>$<span class="output" id="totalinterest"></span></td>
</tr>
<tr>
<th>Sponsors:</th>
<td colspan="2">Apply for your loan with one of these fine lenders:
<div id="lenders"></div>
</td>
</tr>
</tr>
</tr>
</table>
</body>
</html>
效果图：