Using javascript to save base64 String to excel (.xls) file?

Question

I converted a excel file as base64 String as web backend side, and could recover it as excel file as expected with Java code below: String destinationPath = "e:\out1.xls";

        //decode Base64 String to excel file
        FileOutputStream fos = new FileOutputStream(destinationPath); 
        bytes = new sun.misc.BASE64Decoder().decodeBuffer(content);
        fos.write(bytes);
        fos.flush();
        fos.close();

I can response frontend with base64 string, but I always got messy code when I tried to export it as excel (.xls) file using javascript. Below is the code: onClick: function(){

// below is base64 String snippet.
        var base64Str = "0M8R4KGxGuEAAAAAAAAAAAAAAAAAAAAAOwADAP7/CQAGAAAAAAAAAAAAAAABAAAAAAAAAAAAAAAA\r\nEAAA/v///wAAAAD+////AAAAAAEAAAD/////////////////////////////////////////////\r\n////////////////////////////////////////////////////////////////////////////\r\n////////////////////////////////////////////////////////////////////////////\r\n////////////////////////////////////////////////////////////////////////////\r\n////////////////////////////////////////////////////////////////////////////\r\n////////////////////////////////////////////////////////////////////////////\r\n////////////////////////////////////////////////////////////////////////////\r\n//////////////////////////////////////////////////////////////////////////9S\r\nAG8AbwB0ACAARQBuAHQAcgB5AAAAAAAAAAAAAA .........";

        base64Str = base64Str.replace(/\\r\\n/g,"");

        var decoder = Ext.util.Base64.decode(fileStr);
        testApp.view.main.FileExport.saveAs(decoder,'SHANGTOUDI_JF_20180320_0800.xls','UTF-8');
    }

Need to say, I can use this method to export txt file successfully. Any suggestion are appreciated.