i have made my calculator but does not function

[zabihjan]

could someone let me where i am going wrong?
thank you

<script language="javascript" type="text/javascript">
<!-- Begin
function Loan() {
var loanAmount;
var IR;
var years;
var monthPay;
var month;
switch(document.calcform.startm.value) {
case "january":
document.calcform.startm.value="0";
parseInt(document.calcform.startm.value)
break;
case "february":
document.calcform.startm.value="1";
parseInt(document.calcform.startm.value)
break;
case "march":
document.calcform.startm.value="2";
parseInt(document.calcform.startm.value);
break;
case "april":
document.calcform.startm.value="3";
parseInt(document.calcform.startm.value);
break;
case "may":
document.calcform.startm.value="4";
parseInt(document.calcform.startm.value);
break;
case "june":
document.calcform.startm.value="5";
parseInt(document.calcform.startm.value);
break;
case "july":
document.calcform.startm.value="6";
parseInt(document.calcform.startm.value);
break;
case "august":
document.calcform.startm.value="7";
parseInt(document.calcform.startm.value);
break;
case "september":
document.calcform.startm.value="8";
parseInt(document.calcform.startm.value);
break;
case "october":
document.calcform.startm.value="9";
parseInt(document.calcform.startm.value);
break;
case "november":
document.calcform.startm.value="10";
parseInt(document.calcform.startm.value);
break;
case "december":
document.calcform.startm.value="11";
parseInt(document.calcform.startm.value);
break;
default:
document.calcform.startm.value="0";
parseInt(document.calcform.startm.value);
break;
}
loanAmount = parseFloat(document.calcform.price.value);
IR = parseFloat(document.calcform.interest.value);
years = parseFloat(document.calcform.time.value);
loanAmount = loanAmount + int;
month = years * 12;
interestRate = IR / 12;
negmonth =- 1 * month;
var bottom = 1 - (Math.pow(interestRate+1,negmonth));
var top = interestRate;
var mid = top / bottom;
var dec;
document.calcform.monthPay.value = (loanAmount * mid);
monthPay = document.calcform.monthPay.value;
document.calcform.monthPay.value = eval(monthPay);
loanAmount = loanAmount - document.calcform.monthPay.value;
document.calcform.month.value = month;
}
function convert() {
var startmonth = new String();
startmonth = document.calcform.startm.value;
document.calcform.startm.value = startmonth.toLowerCase();
}
function changenum() {
document.calcform.startyear.value=parseInt(document.calcform.startyear.value);
}
var d = new Date;
function getFieldValue (strFieldName) {
var strFieldValue;
var objRegExp = new RegExp(strFieldName + "=([^&]+)","gi");
if (objRegExp.test(location.search))
strFieldValue = unescape(RegExp.£1);
else strFieldValue="";
return strFieldValue;
}
function Currency(money1) {
var money = new String(money1);
var decimal = money.indexOf(".",[0]);
var money = money.substring(0,decimal+3);
var money1 = parseFloat(money);
return money;
}
var startmonth;
startmonth = getFieldValue("startm");
var monthofpayment = new String;
month = getFieldValue("month");
monthPay = getFieldValue("monthPay");
loanAmount = getFieldValue("price");
Ir = getFieldValue("interest");
Ir = Ir/12;
year = getFieldValue("startyear")
function check() {
var price = document.calcform.price.value;
if(price.indexOf(",") >= 0) {
alert("Please do not put commas in the Price number")
   }
}
//  End -->
</script>
<center>
<form method="calc.loan">
<table border=0>
<tr>
<td valign="top">
Anticipated Amount of Loan
<br>
</td>
<td valign="top">
£
<input name=price onblur="check()" size=10>
</td>
</tr>
<tr>
</tr>
<tr>
<td>
Anticipated Interest Rate(.00 format)
<br>
</td>
<td>
<input name=interest size=10>
<br>
</td>
<tr>
<td>
Anticipated length of loan, in years.
<br>
</td>
<td>
<input name="time" size=10>
<br>
</tr>
<tr>
<td>
Start Date of Loan (month and year)
</td>
<td>
<input name=startm onblur="convert()" size=10>
<input name=startyear onblur="changenum()" size=10>
</td>
</td>
</tr>
<input name=month type=hidden>
<input name=monthPay type=hidden>
<tr>
<td>
<input type=submit value="Calculate Loan" onmouseover="check()" onclick="Loan()">
</td>
</tr>
</table>
</form>
<table border=0>
<tr>
<td>
Loan Price
</td>
<td>
£
<script language="javascript">
<!--
document.write(getFieldValue("price"));
//-->
</script>
</td>
</tr>
<td>
Monthly Payment
</td>
<td>
£
<script language="javascript">
<!--
var payment = getFieldValue("monthPay");
payment=Currency(payment);
document.write(payment);
//-->
</script>
</td>
</tr>
<tr>
<td>
# of years
</td>
<td>
<script language="javascript">
<!--
document.write(month/12);
-->
</script>
</td>
</tr>
</table>
<table border="1" >
<tr>
<th width="125">
Month/Year
</th>
<th width="125">
Interest
</th>
</tr>
<script language="JavaScript">
<!--
var halfmonth;
halfmonth = month / 2;
for(y = 0; y <= month; y++) {
var interest = loanAmount * Ir;
var principal = monthPay - interest;
loanAmount = loanAmount - principal;
interest = Currency(interest);
principal = Currency(principal);
if(startmonth > 11) {
startmonth = 0;
year++;
}
switch(startmonth.toString()) {
case "0":monthofpayment="January";
break;
case "1":monthofpayment="February";
break;
case "2":monthofpayment="March";
break;
case "3":monthofpayment="April";
break;
case "4":monthofpayment="May";
break;
case "5":monthofpayment="June";
break;
case "6":monthofpayment="July";
break;
case "7":monthofpayment="August";
break;
case "8":monthofpayment="September";
break;
case "9":monthofpayment="October";
break;
case "10":monthofpayment="November";
break;
case "11":monthofpayment="December";
}
document.write("<tr><td width='125'>")
document.write(monthofpayment+" "+year+"</td>"+"<td width='125'>"+"£"+interest+"</td><td width='125'>"+"£"+principal);
document.write("</td></tr>");
startmonth++;
}
//-->
</script>
</table>
</center>

[jmrker]

You have numerous errors.
Take a look a the error console for some starting help.

Also some useless code, for example:
You have ...

 switch(document.calcform.startm.value) {
  case "january"   : document.calcform.startm.value="0"; parseInt(document.calcform.startm.value); break;
  case "february"  : document.calcform.startm.value="1"; parseInt(document.calcform.startm.value); break;
...

Should look like this ...

 switch(document.calcform.startm.value) {
  case "january"   : document.calcform.startm.value="0"; break;
  case "february"  : document.calcform.startm.value="1"; break;
 ...

or better yet...

  var tmp = document.calcform.startm.value;
  var months = ['january','february','march','april','may','june',
                'july','august','september','october','november','december'];
  var fnd = 0;  // used as default value
  for (var i=0; i<months.length; i++) { if (months[i] == tmp) { fnd = i; } }
  document..calcform.startm.value = fnd;
 ...

Note also, it is not clear to enter the month as a string rather than a number like 1...12 in your site.

[Philip M]

Also very old code. <script language=javascript> is long deprecated. Use <script type = "text/javascript"> instead (in fact also deprecated but still necessary for IE<9).
The <!-- and //--> comment (hiding) tags have not been necessary since IE3 (i.e. since September 1997). If you see these in some published script it is a warning that you are looking at ancient and perhaps unreliable code.