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1.2.1 Example: A JavaScript Loan Calculator

This chapter ends with an extended example that puts many of these techniques to-
gether and shows what real-world client-side JavaScript (plus HTML and CSS) pro-
grams look like. Example 1-1 lists the code for the simple loan payment calculator
application pictured in Figure 1-2.


It is worth reading through Example 1-1 carefully. You shouldn’t expect to understand
everything, but the code is heavily commented and you should be able to at least get

the big-picture view of how it works. The example demonstrates a number of core
JavaScript language features, and also demonstrates important client-side JavaScript
techniques:
• How to find elements in a document.
• How to get user input from form input elements.
• How to set the HTML content of document elements.
• How to store data in the browser.
• How to make scripted HTTP requests.
• How to draw graphics with the <canvas> element.
Example 1-1. A loan calculator in JavaScript
<!DOCTYPE html>   
<html>
<head>
<title>JavaScript Loan Calculator</title>
<style> /* This is a CSS style sheet: it adds style to the program output */
.output { font-weight: bold; }           /* Calculated values in bold */
#payment { text-decoration: underline; } /* For element with id="payment" */
#graph { border: solid black 1px; }      /* Chart has a simple border */
th, td { vertical-align: top; }          /* Don't center table cells */
</style>
</head>
<body>
<!--
  This is an HTML table with <input> elements that allow the user to enter data
  and <span> elements in which the program can display its results.
  These elements have ids like "interest" and "years". These ids are used
  in the JavaScript code that follows the table. Note that some of the input
  elements define "onchange" or "onclick" event handlers. These specify strings
  of JavaScript code to be executed when the user enters data or clicks.
-->
<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>
</table>
<!-- The rest of this example is JavaScript code in the <script> tag below -->
<!-- Normally, this script would go in the document <head> above but it -->
<!-- is easier to understand here, after you've seen its HTML context. -->
<script>
"use strict"; // Use ECMAScript 5 strict mode in browsers that support it
/*
 * This script defines the calculate() function called by the event handlers
 * in HTML above. The function reads values from <input> elements, calculates
 * loan payment information, displays the results in <span> elements. It also
 * saves the user's data, displays links to lenders, and draws a chart.
 */
function calculate() {
    // Look up the input and output elements in the document
    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");
    
    // Get the user's input from the input elements. Assume it is all valid.
    // Convert interest from a percentage to a decimal, and convert from
    // an annual rate to a monthly rate. Convert payment period in years
    // to the number of monthly payments.
    var principal = parseFloat(amount.value);
    var interest = parseFloat(apr.value) / 100 / 12;
    var payments = parseFloat(years.value) * 12;
    // Now compute the monthly payment figure.
    var x = Math.pow(1 + interest, payments);   // Math.pow() computes powers
    var monthly = (principal*x*interest)/(x-1);
    // If the result is a finite number, the user's input was good and
    // we have meaningful results to display
    if (isFinite(monthly)) {
        // Fill in the output fields, rounding to 2 decimal places
        payment.innerHTML = monthly.toFixed(2);
        total.innerHTML = (monthly * payments).toFixed(2);
        totalinterest.innerHTML = ((monthly*payments)-principal).toFixed(2);
        // Save the user's input so we can restore it the next time they visit
        save(amount.value, apr.value, years.value, zipcode.value);
        
        // Advertise: find and display local lenders, but ignore network errors
        try {      // Catch any errors that occur within these curly braces
            getLenders(amount.value, apr.value, years.value, zipcode.value);
        }
        catch(e) { /* And ignore those errors */ }
        // Finally, chart loan balance, and interest and equity payments
        chart(principal, interest, monthly, payments);
    }
    else {  
        // Result was Not-a-Number or infinite, which means the input was
        // incomplete or invalid. Clear any previously displayed output.
        payment.innerHTML = "";        // Erase the content of these elements
        total.innerHTML = ""
        totalinterest.innerHTML = "";
        chart();                       // With no arguments, clears the chart
    }
}
// Save the user's input as properties of the localStorage object. Those
// properties will still be there when the user visits in the future
// This storage feature will not work in some browsers (Firefox, e.g.) if you 
// run the example from a local file:// URL.  It does work over HTTP, however.
function save(amount, apr, years, zipcode) {
    if (window.localStorage) {  // Only do this if the browser supports it
        localStorage.loan_amount = amount;
        localStorage.loan_apr = apr;
        localStorage.loan_years = years;
        localStorage.loan_zipcode = zipcode;
    }
}
// Automatically attempt to restore input fields when the document first loads.
window.onload = function() {
    // If the browser supports localStorage and we have some stored data
    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;
    }
};
// Pass the user's input to a server-side script which can (in theory) return
// a list of links to local lenders interested in making loans.  This example
// does not actually include a working implementation of such a lender-finding
// service. But if the service existed, this function would work with it.
function getLenders(amount, apr, years, zipcode) {
    // If the browser does not support the XMLHttpRequest object, do nothing
    if (!window.XMLHttpRequest) return;
    // Find the element to display the list of lenders in
    var ad = document.getElementById("lenders");
    if (!ad) return;                            // Quit if no spot for output 
    // Encode the user's input as query parameters in a URL
    var url = "getLenders.php" +                // Service url plus
        "?amt=" + encodeURIComponent(amount) +  // user data in query string
        "&apr=" + encodeURIComponent(apr) +
        "&yrs=" + encodeURIComponent(years) +
        "&zip=" + encodeURIComponent(zipcode);
    // Fetch the contents of that URL using the XMLHttpRequest object
    var req = new XMLHttpRequest();        // Begin a new request
    req.open("GET", url);                  // An HTTP GET request for the url
    req.send(null);                        // Send the request with no body
    // Before returning, register an event handler function that will be called
    // at some later time when the HTTP server's response arrives. This kind of 
    // asynchronous programming is very common in client-side JavaScript.
    req.onreadystatechange = function() {
        if (req.readyState == 4 && req.status == 200) {
            // If we get here, we got a complete valid HTTP response
            var response = req.responseText;     // HTTP response as a string
            var lenders = JSON.parse(response);  // Parse it to a JS array
            // Convert the array of lender objects to a string of HTML
            var list = "";
            for(var i = 0; i < lenders.length; i++) {
                list += "<li><a href='" + lenders[i].url + "'>" +
                    lenders[i].name + "</a>";
            }
            // Display the HTML in the element from above.
            ad.innerHTML = "<ul>" + list + "</ul>"; 
        }
    }
}
// Chart monthly loan balance, interest and equity in an HTML <canvas> element.
// If called with no arguments then just erase any previously drawn chart.
function chart(principal, interest, monthly, payments) {
    var graph = document.getElementById("graph"); // Get the <canvas> tag
    graph.width = graph.width;  // Magic to clear and reset the canvas element
    // If we're called with no arguments, or if this browser does not support
    // graphics in a <canvas> element, then just return now.
    if (arguments.length == 0 || !graph.getContext) return;
    // Get the "context" object for the <canvas> that defines the drawing API
    var g = graph.getContext("2d"); // All drawing is done with this object
    var width = graph.width, height = graph.height; // Get canvas size
    // These functions convert payment numbers and dollar amounts to pixels
    function paymentToX(n) { return n * width/payments; }
    function amountToY(a) { return height-(a * height/(monthly*payments*1.05));}
    // Payments are a straight line from (0,0) to (payments, monthly*payments)
    g.moveTo(paymentToX(0), amountToY(0));         // Start at lower left
    g.lineTo(paymentToX(payments),                 // Draw to upper right
             amountToY(monthly*payments));
    g.lineTo(paymentToX(payments), amountToY(0));  // Down to lower right
    g.closePath();                                 // And back to start
    g.fillStyle = "#f88";                          // Light red
    g.fill();                                      // Fill the triangle
    g.font = "bold 12px sans-serif";               // Define a font
    g.fillText("Total Interest Payments", 20,20);  // Draw text in legend
    // Cumulative equity is non-linear and trickier to chart
    var equity = 0;
    g.beginPath();                                 // Begin a new shape
    g.moveTo(paymentToX(0), amountToY(0));         // starting at lower-left
    for(var p = 1; p <= payments; p++) {
        // For each payment, figure out how much is interest
        var thisMonthsInterest = (principal-equity)*interest;
        equity += (monthly - thisMonthsInterest);  // The rest goes to equity
        g.lineTo(paymentToX(p),amountToY(equity)); // Line to this point
    }
    g.lineTo(paymentToX(payments), amountToY(0));  // Line back to X axis
    g.closePath();                                 // And back to start point
    g.fillStyle = "green";                         // Now use green paint
    g.fill();                                      // And fill area under curve
    g.fillText("Total Equity", 20,35);             // Label it in green
    // Loop again, as above, but chart loan balance as a thick black line
    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);     // The rest goes to equity
        g.lineTo(paymentToX(p),amountToY(bal));    // Draw line to this point
    }
    g.lineWidth = 3;                               // Use a thick line
    g.stroke();                                    // Draw the balance curve
    g.fillStyle = "black";                         // Switch to black text
    g.fillText("Loan Balance", 20,50);             // Legend entry
    // Now make yearly tick marks and year numbers on X axis
    g.textAlign="center";                          // Center text over ticks
    var y = amountToY(0);                          // Y coordinate of X axis
    for(var year=1; year*12 <= payments; year++) { // For each year
        var x = paymentToX(year*12);               // Compute tick position
        g.fillRect(x-0.5,y-3,1,3);                 // Draw the tick
        if (year == 1) g.fillText("Year", x, y-5); // Label the axis
        if (year % 5 == 0 && year*12 !== payments) // Number every 5 years
            g.fillText(String(year), x, y-5);
    }
    // Mark payment amounts along the right edge
    g.textAlign = "right";                         // Right-justify text
    g.textBaseline = "middle";                     // Center it vertically
    var ticks = [monthly*payments, principal];     // The two points we'll mark
    var rightEdge = paymentToX(payments);          // X coordinate of Y axis
    for(var i = 0; i < ticks.length; i++) {        // For each of the 2 points
        var y = amountToY(ticks[i]);               // Compute Y position of tick
        g.fillRect(rightEdge-3, y-0.5, 3,1);       // Draw the tick mark
        g.fillText(String(ticks[i].toFixed(0)),    // And label it.
                   rightEdge-5, y);
    }
}
</script>
</body>
</html>























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