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HTML
JavaScript
CSS

<!DOCTYUpdatePE 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>
    <title>JSON to CSV</title>
</head>
<body>
    <p id="heading">JSON to CSV Converter</p>
    <p class="small"><a href="http://jsfiddle.net/sturtevant/AZFvQ/" target="_blank">CSV to JSON Converter</a>
    <hr />
    <p>Paste Your JSON Here:</p>
    <textarea id="json" class="text">[{"Id":1,"UserName":"Sam Smith"},
{"Id":2,"UserName":"Fred Frankly"},
{"Id":1,"UserName":"Zachary Zupers"}]</textarea>
    <br />
    <input id="quote" type="checkbox" checked="true" /> Wrap values in double quotes
    <br />
    <input id="labels" type="checkbox" checked="true" /> Include labels in first row
    <br />
    <button id="convert">Convert to CSV</button>
    &nbsp;&nbsp;
    <button id="download">Download CSV</button>
    <textarea id="csv" class="text"></textarea>
    <p>Based on code posted <a href="http://stackoverflow.com/a/4130939/317" target="_blank">here on StackOverflow</a></p>
</body>
</html>

function JSON2CSV(objArray) {
    var array = typeof objArray != 'object' ? JSON.parse(objArray) : objArray;

    var str = '';
    var line = '';

    if ($("#labels").is(':checked')) {
        var head = array[0];
        if ($("#quote").is(':checked')) {
            for (var index in array[0]) {
                var value = index + "";
                line += '"' + value.replace(/"/g, '""') + '",';
            }
        } else {
            for (var index in array[0]) {
                line += index + ',';
            }
        }

        line = line.slice(0, -1);
        str += line + '\r\n';
    }

    for (var i = 0; i < array.length; i++) {
        var line = '';

        if ($("#quote").is(':checked')) {
            for (var index in array[i]) {
                var value = array[i][index] + "";
                line += '"' + value.replace(/"/g, '""') + '",';
            }
        } else {
            for (var index in array[i]) {
                line += array[i][index] + ',';
            }
        }

        line = line.slice(0, -1);
        str += line + '\r\n';
    }
    return str;
    
}
        
$("#convert").click(function() {
    var json = $.parseJSON($("#json").val());
    var csv = JSON2CSV(json);
    $("#csv").val(csv);
});
    
$("#download").click(function() {
    var json = $.parseJSON($("#json").val());
    var csv = JSON2CSV(json);
    window.open("data:text/csv;charset=utf-8," + escape(csv))
});

#heading { font-size: x-large; font-weight: bold; }
.text { width: 99%; height: 200px; }
.small { font-size: small; }