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Asked: 2026-05-22T00:49:58+00:00

I posted this code a couple days back, but the discussion seemed to wax

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I posted this code a couple days back, but the discussion seemed to wax philosophical about the weaknesses of Javascript (not to mention my own obvious weaknesses as a “programmer”) and then died out without ever clarifying a solution. I’m hoping someone will help rectify that.

Due to a phenomenon apparently known as “floating point math,” Javascript returns an arithmetically inaccurate answer (.00000004 or similar below the correct answer) in the calculator code below. I was advised to round the answer up by “calling math.round() on the variable,” which I think would work fine for my purposes, only my JS kung fu was too weak, and the syntax of doing so in context has thus far eluded me.

Where/how to I make this call? So far all my attempts have failed, even when I thought for sure each would not. I would sure appreciate an answer that takes into account my low-level knowledge of the subject. This has got to be a slam-dunk for somebody out there.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Untitled Document</title>

<script language="javascript">

<!-- Begin Trip Tickets Savings Calc script
function  doMath4() {
    var one = parseInt(document.theForm4.elements[0].value);
    var two = parseInt(document.theForm4.elements[1].value);
    var selection = document.getElementsByName("zonett")[0].value;

    if(selection == "z4"){
        var prodZ4tt = (((one  *   two) * 4.25) *12) - (((one  *   two) * 3.75) *12);
        alert("Your yearly savings if you buy Trip Tickets is $"  +  prodZ4tt  +  ".");
    }
    else if(selection == "z3"){
        var prodZ3tt = (((one  *   two) * 3.75) *12) - (((one  *   two) * 3.35) *12);
        alert("Your yearly savings if you buy Trip Tickets is $"  +  prodZ3tt  +  ".");
    }
    else if(selection == "z2"){
        var prodZ2tt = (((one  *   two) * 3) *12) - (((one  *   two) * 2.8) *12);
        alert("Your yearly savings if you buy Trip Tickets is $"  +  prodZ2tt  +  ".");
    }
    else if(selection == "z1"){
        var prodZ1tt = (((one  *   two) * 2.5) *12) - (((one  *   two) * 2.3) *12);
        alert("Your yearly savings if you buy Trip Tickets is $"  +  prodZ1tt  +  ".");
    }
    else if(selection == "Base"){
        var prodBasett = (((one  *   two) * 1.5) *12) - (((one  *   two) * 1.5) *12);
        alert("Your yearly savings if you buy Trip Tickets is $"  +  prodBasett  +  ".");
    }
}

// End Trip Tickets Savings Calc script -->

</script>
</head>

<body>


<form name="theForm4" class="calcform">
<h2>You Do the Math: Commuter Express Trip Tickets Vs. Cash</h2>
<div class="calcform-content">
    <div class="formrow-calc">
      <div class="calcform-col1">
        <p>Days you commute on Commuter Express monthly:</p>
      </div>
      <div class="calcform-col2">
        <input type="text">
      </div>
      <div class="calcform-col3">&nbsp;</div>
    </div>
    <div class="clear"></div>


    <div class="formrow-calc">
      <div class="calcform-col1">
        <p>Daily boardings on Commuter Express Bus:</p>

        <table border="0" cellspacing="0" cellpadding="0" class="fareexampletable">
            <tr>
              <td colspan="2" class="savingsleft"><p class="ifyouride">EXAMPLE:</p></td>
              </tr>
            <tr>
              <td class="savingsleft"><p><strong>Go to work:</strong></p></td>
              <td class="savingsright"><p>1 time</p></td>
            </tr>
            <tr>
              <td class="savingsleft"><p><strong>Come home from work:</strong></p></td>
              <td class="additionline savingsright"><p>+1 time</p></td>
            </tr>
            <tr>
              <td class="savingsleft"><p><strong>Total:</strong></p></td>
              <td class="savingsright"><p>2 times</p></td>
            </tr>
          </table>
      </div>
      <div class="calcform-col2">
        <input type="text">
      </div>
      <div class="calcform-col3">&nbsp;</div>
    </div>
    <div class="clear"></div>


    <div class="formrow-calc savings-zone">
      <div class="calcform-col1">
        <p>Choose Zone:</p>
      </div>
      <div class="calcform-col2">
        <select name="zonett">
          <option value="Base">Base</option>
          <option value="z1">Zone 1</option>
          <option value="z2">Zone 2</option>
          <option value="z3">Zone 3</option>
          <option value="z4">Zone 4</option>
        </select>
      </div>
    </div>



    <div class="formrow-calc">


          <div class="calcform-col4-ce">


 <button type="submit" onclick="doMath4()" class="btn-submit"><div class="btn-submit"><img src="img/btn_savings.png" alt="Show My Yearly Savings" /></div></button> 


    </div>
    </div>
    <div class="clear">


</div>



</div>
</form>

</body>
</html>
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1 Answer

  1. Editorial Team

    You can write a kludge with Math.round, but that will nuke cents (not sure if this applies in your example, it sort of looks like it could). What you may want is toFixed:

    alert((102.000000004).toFixed(2)); // alerts "102.00"

    This does NOT round in IE, but from what I can tell this will not matter in your case.

    (docs: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Number/toFixed , http://www.codingforums.com/showthread.php?t=102421 , http://msdn.microsoft.com/en-us/library/sstyff0z.aspx )

    Edit: intended to put this in the comments, but markdown doesn’t work there? Still getting used to SO…

    Anyhow, you can also use a kludge with Math.round if you prefer:

    alert(Math.round(val * 100) / 100);

    Which may still display the silly float issues, for which you could use another .toFixed (and now you’re 100% sure there’s no leftover decimal places you’re not rounding properly):

    alert((Math.round(val * 100) / 100).toFixed(2));

    Edit: to integrate:

    Add this at the top of your script block:

    function roundNicely(val) {
     return (Math.round(val * 100) / 100).toFixed(2);
}

    Then in your alerts, wrap the value prodBasett in the function call: roundNicely(prodBasett) (and similar for the other alerts)

    Does that make sense?

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