function onKeyDown(e) {
    if (e.keyCode === 9) { // tab key
        e.preventDefault();  // this will prevent us from tabbing out of the editor

        // now insert four non-breaking spaces for the tab key
        var editor = document.getElementById("editor");
        var doc = editor.ownerDocument.defaultView;
        var sel = doc.getSelection();
        var range = sel.getRangeAt(0);

        var tabNode = document.createTextNode("\u00a0\u00a0\u00a0\u00a0");
        range.insertNode(tabNode);

        range.setStartAfter(tabNode);
        range.setEndAfter(tabNode); 
        sel.removeAllRanges();
        sel.addRange(range);
    }
}