http://bit.ly/Create-XLSX-JavaScript-SharePoint

Create XLSX in JavaScript

// By Vincent Rothwell
// http://blog.thekid.me.uk
//
// Taken from blog post http://bit.ly/Create-XLSX-JavaScript-SharePoint

function createSpreadsheetData(number) {
    var openedSpreadsheet = new openXml.OpenXmlPackage(blankSpreadsheet);

    var workbookPart = openedSpreadsheet.workbookPart();
    var wbXDoc = workbookPart.getXDocument();
    var sheetElement = wbXDoc.root.element(openXml.S.sheets)
                                  .elements(openXml.S.sheet)
                                  .firstOrDefault(function (sh) {
                                       return sh.attribute("name").value === 'Sheet1';
                                  });

    if (sheetElement) {
        var id = sheetElement.attribute(openXml.R.id).value;
        var worksheetPart = workbookPart.getPartById(id);
        var wsXDoc = worksheetPart.getXDocument();
        var table = wsXDoc.descendants(openXml.S.sheetData).firstOrDefault();

        var newRow = new Ltxml.XElement(openXml.S.row,
                                        new Ltxml.XAttribute("r", 1));
        table.add(newRow);

        var cell = new Ltxml.XElement(openXml.S.c,
                                      new Ltxml.XAttribute("r", "A1"),
                                      new Ltxml.XElement(openXml.S.v, number));
        newRow.add(cell);
    }

    return openedSpreadsheet.saveToBase64();
}