使用Javascript的计算器HTML表单

Question

我有一点问题。这是我的代码，我不知道我做错了什么。我尝试完成一个简单的HTML表单来计算文本输入html元素中的给定数字。

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function rebate(){
  var rebate_a,rebate_b,total,final;

  var a = document.myform.rebatenum.value;

  if( a <= 3000 )
  {
    final = a;
  }
  else if ( (a > 3000) && (a <= 10000 ) )
  { 
    rebate_a = (a-3000) * 0.02;
    total = rebate_a;
    final = a - rebate_a;
  }
  else if ( (a > 10000) && (a <= 30000 ) )
  { 
    rebate_a = (7000 * 0.02) + (a-10000) * 0.03;
    total = rebate_a;
    final = a - rebate_a;
  }
  else if ( (a > 30000) && (a <= 40000 ) )
  { 
    rebate_a = (7000 * 0.02) + (20000 * 0.03) + (a-30000) * 0.05;
    total = rebate_a;
    final = a - rebate_a;
  }
  else if( (a >= 40000) && (a < 50000 ))
  {
    rebate_a = (7000 * 0.02) + (20000 * 0.03) + (10000 * 0.05) + (a-40000) * 0.06;
    rebate_b = 25;
    total = rebate_a + rebate_b;
    final = a - total;
  }
  else
  {
    document.myform.final.value = "20";
  }

  document.getElementById("demo").innerHTML = a;
  document.myform.final.value = final;
}

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<form name="myform">
<table border="0">
<tr>
<td>Initial Price</td>
    <td><input name="rebatenum" type="text" size="12" onchange="rebate();"></td>
</tr>
<tr>
    <td colspan="2"><input type="button" value="Υπολογισμός" onclick="rebate();"></td>
</tr>
<tr>
        <td colspan="2"></td>
</tr>
<tr>
     <td>Final Price after rebate:</td>
     <td><input type="text" name="final" size="12"></td>
</tr>
</table>
</form>
    <p id="demo"></p>

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你也可以在这里找到!!!

http://jsfiddle.net/pf7Lysqt/3/