function saveTextAsFile()
{
    var textToWrite = //Your text input;
    var textFileAsBlob = new Blob([textToWrite], {type:'text/plain'});
    var fileNameToSaveAs = //Your filename;

    var downloadLink = document.createElement("a");
    downloadLink.download = fileNameToSaveAs;
    downloadLink.innerHTML = "Download File";
    if (window.webkitURL != null)
    {
        // Chrome allows the link to be clicked
        // without actually adding it to the DOM.
        downloadLink.href = window.webkitURL.createObjectURL(textFileAsBlob);
    }
    else
    {
        // Firefox requires the link to be added to the DOM
        // before it can be clicked.
        downloadLink.href = window.URL.createObjectURL(textFileAsBlob);
        downloadLink.onclick = destroyClickedElement;
        downloadLink.style.display = "none";
        document.body.appendChild(downloadLink);
    }

    downloadLink.click();
}

My text area:<br />
<textarea rows='10' cols='80' id='myTextArea' ></textarea>

<br /><br />

Download button: <br />
<input value='download' type='button'
onclick='doDL(document.getElementById("myTextArea").value)' />


<script type='text/javascript'>
function doDL(s){
    function dataUrl(data) {return "data:x-application/text," + escape(data);}
    window.open(dataUrl(s));
}
</script>

function doDownload(str) {
              function dataUrl(data) {
                return "data:x-application/xml;charset=utf-8," + escape(data);
              }
              var downloadLink = document.createElement("a");
              downloadLink.href = dataUrl(str);
              downloadLink.download = "foo.xml";

              document.body.appendChild(downloadLink);
              downloadLink.click();
              document.body.removeChild(downloadLink);
            }