最近开始阅读《JavaScript权威指南》第六版，这次阅读的主要目地是进一步把基础打牢，对以前的学习复习和检查。
第一章是JavaScript概述，没什么好说的，只把本章最后一个综合示例（贷款计算器）动手敲了一遍：
代码如下：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"/>
<title>Demo</title>
<style type="text/css">
.output{
font-weight:bold;
}
#payment{
text-decoration:underline;
}
#graph{
border:1px solid black;
}
th,td{
vertical-align:top;
}
</style>
</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($):</td>
<td><input type="text" id="amount" onchange="calculate();"/></td>
<td rowspan=8>
<canvas id="graph" width="400" height="250"></canvas>
</td>
</tr>
<tr>
<td>Annual interest(%):</td>
<td><input type="text" id="apr" onchange="calculate();"/></td>
</tr>
<tr>
<td>Repayment penod(year):</td>
<td><input type="text" id="years" onchange="calculate();"/></td>
</tr>
<tr>
<td>Zipcode(to find lenders):</td>
<td><input type="text" id="zipcode" onchange="calculate();"/></td>
</tr>
<tr>
<td>Approximate Payments:</td>
<td><input type="button" value="Calculate" onclick="calculate();"/></td>
</tr>
<tr>
<td>Monthly payment:</td>
<td>$<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>
</table>
<script>
"use strict";
function $(id){
return document.getElementById(id);
}
function calculate(){
var amount=$("amount"),
apr=$("apr"),
years=$("years"),
zipcode=$("zipcode"),
payment=$("payment"),
total=$("total"),
totalinterest=$("totalinterest"),
principal=parseFloat(amount.value),
interest=parseFloat(apr.value)/100/12,
payments=parseFloat(years.value)*12,
x=Math.pow(1+interest,payments),
monthly=(principal * x * interest)/(x-1);
if(isFinite(monthly)){
payment.innerHTML = monthly.toFixed(2);
total.innerHTML = (monthly * payments).toFixed(2);
totalinterest.innerHTML= ((monthly * payments) - principal).toFixed(2);
save(amount.value,apr.value,years.value,zipcode.value);
try{
getLenders(amount.value,zpr.value,years.value,zipcode.value);
} catch(e){
}
chart(principal,interest,monthly,payments);
} else {
payment.innerHTML="";
total.innerHTML="";
totalinterest.innerHTML="";
chart();
}
}
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){
$("amount").value = localStorage.loan_amount;
$("apr").value = localStorage.loan_apr;
$("years").value = localStorage.loan_years;
$("zipcode").value = localStorage.loan_zipcode;
}
}
function getLenders(amount,zpr,years,zipcode){
if(window.XMLHttpRequest)
return;
var ad=$("lenders");
if(!ad)
return;
var url="genLenders.php"+
"?amt="+encodeURIComponent(amount)+
"&apr="+encodeURIComponent(zpr)+
"&yrs="+encodeURIComponent(years)+
"&zip="+encodeURIComponent(zipcode);
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, j = lenders.length; i<j; i++){
list+="<li><a href='"+lenders[i].url+"'>" + lenders[i].name + "</a></li>";
}
ad.innerHTML = "<ul>" + list + "</ul>";
}
}
}
function chart(principal, interest, monthly, payments){
var graph=$("graph");
//graph.width=graph.wdith;
if(arguments.length == 0 || !graph.getContext)
return;
var g=graph.getContext("2d"),
width=graph.width,
height=graph.height;
g.clearRect(0, 0, width, 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(monthly * payments));
g.lineTo(paymentToX(payments),amountToY(0));
g.closePath();
g.fillStyle="#888";
g.fill();
g.fillText("Total Interest Payments",20,20);
var equity=0;
g.beginPath();
g.moveTo(paymentToX(0),amountToY(0));
for(var p=1;p<= payments; p++){
var thisMonthsInterest = (principal - 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 = principal;
g.beginPath();
g.moveTo(paymentToX(0),amountToY(bal));
for(var p=1;p<=payments;p++){
var thisMonthsInterest=bal*interest;
bal-=(monthly - thisMonthsInterest);
g.lineTo(paymentToX(p),amountToY(bal));
}
g.lineWidth=3;
g.stroke();
g.fillStyle="black";
g.fillText("Loan Black",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(String(year),x,y-5);
}
}
g.textAlign="right";
g.textBaseLine="middle";
var ticks=[monthly * payments,principal];
var rightEdge = paymentToX(payments);
for(var i=0;i< ticks.length;i++){
var y=amountToY(ticks[i]);
g.fillRect(rightEdge-3,y-0.5,3,1);
g.fillText(String(ticks[i].toFixed(0)),rightEdge-5,y);
}
}
</script>
</body>
</html>
运行效果:
要注意的是书上的代码中有一行：
graph.width=graph.wdith;//用一种巧妙的手法清除并重置画布
这一行代码经测试不能达到清除并重置画布的效果，反而使右达的图像只是一条垂直的黑线，所以我换成了:
g.clearRect(0, 0, width, height);
也没找到勘误表，不在道这个地方是不是有印刷错误，如果有知道的朋烦请告知。:)