贷款计算器Javascript代码问题？

Question

我正在使用Javascript代码为我的网站创建一个贷款计算器，网站为https://www.usawebcash.com/missouri-installment-loans。我在Javascript中编写这个贷款计算器的问题。我想要这个代码的帮助，代码如下：

<script language="JavaScript">
  <!--
  function showpay() {
    if ((document.calc.loan.value == null || document.calc.loan.value.length == 0) ||
      (document.calc.months.value == null || document.calc.months.value.length == 0) ||
      (document.calc.rate.value == null || document.calc.rate.value.length == 0)) {
      document.calc.pay.value = "Incomplete data";
    } else {
      var princ = document.calc.loan.value
      var term = document.calc.months.value
      var intr = document.calc.rate.value / 1200
      document.calc.pay.value = princ * intr / (1 - (Math.pow(1 / (1 + intr), term)));
    }

    // payment = principle * monthly interest/(1 - (1/(1+Interest of Month)*Months))

  }
  // -->
</script>


Calculate button. Clicking the Reset button will clear all values
<p>
  <center>
    <form name=calc method=POST>
      <table width=60% border=0>
        <tr>
          <th bgcolor="#aaaaaa" width=50%><font color=blue>Description</font>
          </th>
          <tr bgcolor="#aaaaaa" width=50%><font color=blue>Data Entry</font>
            </th>
          </tr>
          <tr>
            <td bgcolor="#eeeee">Loan Amount</td>
            <td bgcolor="#aaeeaa" align=right>
              <input type=text name=loan size=10>
            </td>
          </tr>
          <tr>
            <td bgcolor="#eeeee">Loan Length in Months</td>
            <td bgcolor="#aaeeaa" align=right>
              <input type=text name=months size=10>
            </td>
          </tr>
          <tr>
            <td bgcolor="#eeeee">Interest Rate</td>
            <td bgcolor="#aaeeaa" align=right>
              <input type=text name=rate size=10>
            </td>
          </tr>
          <tr>
            <td bgcolor="#eeeee">Monthly Payment</td>
            <td bgcolor="#eeaaaa" align=right><em>Calculated</em> 
              <input type=text name=pay size=10>
            </td>
          </tr>
          <tr>
            <td bgcolor="#aaeeaa" align=center>
              <input type=button onClick='showpay()' value=Calculate>
            </td>
            <td bgcolor="#eeeeaa" align=center>
              <input type=reset value=Reset>
            </td>
          </tr>
      </table>
    </form>
    <font size=1>Enter only numeric values <br>
    Non-numeric values will cause errors are not allowed
    
    <p align="center"><font face="arial" size="-2">This free script provided by</font>
    <br>
    <font face="arial, helvetica" size="-2"><a href="http://javascriptkit.com">JavaScript
    Kit</a></font>
</p>