Edit in JSFiddle

$(function () {
    // main source
    // from jquery 1.7.2
    var isNumeric = function (val) {
        return !isNaN(parseFloat(val)) && isFinite(val);
    };
    // has problems with "361111111111111111 / 86".
    // real answer = 4.19896640826873384884e15
    // javascript gives = 4198966408268734.00000000000000000000
    // decimalExpansion gives 4198966408268734.7674418604651
    /**
     * @author Larry Battle <http://bateru.com/news/contact-me>
     * @date May 16, 2012
     * @license MIT and GPLv3
     */
    //decimalExpansion returns a string representation of a divided by b to a fixed length.
    // All the paramaters must be whole numbers.
    // Example: decimalExpansion( 1, 3, 3 ) === "0.333"
    var decimalExpansion = function (top, bottom, decLength) {
        if (!isNumeric(top) || !isNumeric(bottom) || !isNumeric(decLength) || !bottom) {
            return null;
        }
        var sign = ((top * bottom) != Math.abs(top * bottom)) ? "-" : "";
        top = Math.abs(top);
        bottom = Math.abs(bottom);
        decLength = Math.abs(decLength);
        
        var result = Math.floor(top / bottom),
        remainder = top % bottom,
        maxDecimal = 100,
        i = Math.min(Math.max(0, decLength), maxDecimal) + 1;
        
        if (1 < i) {
            result += ".";
            while (i--) {
                top = remainder * 10;
                remainder = top % bottom;
                result += "" + Math.floor(top / bottom);
            }
            result = result.replace(/(\d)(\d)$/, function (match, a, b) {
                    return +b > 4 ? +a + 1 : a;
                });
        }
        return sign + result;
    };
    // test cases.
    test("test for invalid inputs", function () {
        equal(decimalExpansion("s", 3, 1), null);
        equal(decimalExpansion(1, "s", 1), null);
        equal(decimalExpansion(1, "s", -1), null);
        equal(decimalExpansion(1, 0, 1), null);
    });
    test("test whole numbers result (decLength = 0)", function () {
        equal(decimalExpansion(0, 3, 0), "0");
        equal(decimalExpansion(3, 3, 0), "1");
        equal(decimalExpansion(9, 3, 0), "3");
        equal(decimalExpansion(-9, -3, 0), "3");
        equal(decimalExpansion(-9, 3, 0), "-3");
        equal(decimalExpansion(9, -3, 0), "-3");
    });
    test("test postive numbers", function () {
        equal(decimalExpansion(1, 3, 1), "0.3");
        equal(decimalExpansion(1, 3, 2), "0.33");
        equal(decimalExpansion(1, 3, 3), "0.333");
        equal(decimalExpansion(1, 3, 200), decimalExpansion(1, 3, 100));
        equal(decimalExpansion(1, 33, 4), "0.0303");
        equal(decimalExpansion(4, 3, 3), "1.333");
        equal(decimalExpansion(40, 3, 3), "13.333");
    });
    test("test negative numbers", function () {
        equal(decimalExpansion(1, -3, 1), "-0.3");
        equal(decimalExpansion(-1, 3, 2), "-0.33");
        equal(decimalExpansion(1, -3, 3), "-0.333");
        equal(decimalExpansion(-1, 33, 4), "-0.0303");
        equal(decimalExpansion(-4, 3, 3), "-1.333");
        equal(decimalExpansion(-40, 3, 3), "-13.333");
    });
    test("test rounding", function () {
        equal(decimalExpansion(100, 11, 3), "9.091");
        equal(decimalExpansion(1, 3, 0), "0");
        equal(decimalExpansion(1, 10, 0), "0");
        equal(decimalExpansion(10, 18, 0), "0");
        equal(decimalExpansion(1, 7, 0), "0");
        equal(decimalExpansion(5, 49, 2), "0.10");
        equal(decimalExpansion(5, 49, 2), "0.10");
    });
    // user controls events.
    var updateOutput = function () {
        var a = $("#input-numerator").val() || 1;
        var b = $("#input-denominator").val() || 1;
        var decLen = $("#decimalLength").val() || 1;
        $("#javascript-output").html((a / b).toFixed(Math.min(Math.max(decLen, 0), 20)));
        var val = decimalExpansion(a, b, decLen) || "You must type in a number.";
        $("#decimalExpansion-output").html(val);
    };
    var rNum = function (val) {
        return Math.floor((val || 1) * Math.random());
    };
    var generateExample = function () {
        $("#input-numerator").val(rNum(100));
        $("#input-denominator").val(rNum(100));
        $("#decimalLength").val(rNum(100));
        updateOutput();
    };
    $("#exampleButton").click(generateExample);
    $(".inputs").keyup(updateOutput);
    generateExample();
});

<!--
    /**
    * @author Larry Battle <http://bateru.com/news/contact-me>
    * @date May 16, 2012
    * @license MIT and GPLv3
    */
-->
<!DOCTYPE HTML PUBLIC>
<html>
    <head>
        <link rel="stylesheet" href="http://code.jquery.com/qunit/git/qunit.css" type="text/css" media="screen" />
    </head>
    <body>

    <fieldset>
        <legend>Javascript Demo: toFixed() vs decimalExpansion()</legend>            
            Fraction:
            <input type="text" class="inputs" name="input-numerator" id="input-numerator" size="4"> / <input type="text" class="inputs" id="input-denominator" size="4"><br/><br/>
            
            Decimal Length:
            <input type="text" class="inputs" id="decimalLength">
            <br/>
            <dl>
                <dt>Output from ( a/b ).toFixed( decimalLength )</dt>
                <dd><textarea cols=45 id="javascript-output"></textarea><br/></dd>
            </dl>
            <dl>
                <dt>Output from decimalExpansion( a, b, decimalLength )</dt>
                <dd><textarea  cols=45 id="decimalExpansion-output"></textarea><br/></dd>
            </dl>
            
            <input type="button" id="exampleButton" value="Generate Random Example"/>
            <br/>
        </div>
        For more information visit: <a href="http://bateru.com/news/?s=decimal+expansion">Bateru.com/news/</a>                
        </fieldset>

        <h1 id="qunit-header">Decimal Expansion example</h1>
        <h2 id="qunit-banner"></h2>
        <div id="qunit-testrunner-toolbar"></div>
        <h2 id="qunit-userAgent"></h2>
        <ol id="qunit-tests"></ol>
        <div id="qunit-fixture">test markup, will be hidden</div>
        <script src="http://code.jquery.com/jquery-latest.js"></script>
        <script type="text/javascript" src="http://code.jquery.com/qunit/git/qunit.js"></script>  
    </body>
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