Edit in JSFiddle

/**
 * Workarounds for browser like ie8 that doesn't supports Blobs...
 */
function SaveContents() {
    if (document.execCommand) {
        var oWin = window.open("about:blank", "_blank");
        //To add a new line, you need to use <br>
        oWin.document.writeln("First Line<br>");
        oWin.document.writeln("Second Line<br>");
        oWin.document.writeln("Third Line");
        oWin.document.close();
        
        //Need to specify the filename that we are going to set here
        var success = oWin.document.execCommand('SaveAs', true, "output.txt")
        oWin.close();
        if (!success) {}
    }
}

/**
 * Most of the Blob related codes are mainly here but if the browser doesn't supports Blob, then you would redirect to the function SaveContents()
 */
function saveTextAsFile() {
    //For Blobs, use \r\n for new line
    var textToWrite = "First Line\r\n";
    textToWrite += "Second Line\r\n";
    textToWrite += "Third Line";
    
    //Check if browser supports Blob
    if (window.Blob) {
        var textFileAsBlob = new Blob([textToWrite], {
            type: 'text/plain'
        });
        
        //Need to specify the filename that we are going to set here
        var fileNameToSaveAs = "output.txt";
        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);
        }
        if (navigator.msSaveBlob) {
            navigator.msSaveBlob(textFileAsBlob, fileNameToSaveAs);
        } else {
            downloadLink.click();
        }
    } else {
        SaveContents();
    }
}

//Remove the downloadable link
function destroyClickedElement(event) {
    document.body.removeChild(event.target);
}

<button type="button" onclick="saveTextAsFile();">Save</button>