function working in Chrome but not working in IE 11

$('#editable').on('keydown', function (event) {
    if (window.getSelection && event.which == 8) { // backspace
        // fix backspace bug in FF
        // https://bugzilla.mozilla.org/show_bug.cgi?id=685445
        var selection = window.getSelection();
        if (!selection.isCollapsed || !selection.rangeCount) {
            return;
        }

        var curRange = selection.getRangeAt(selection.rangeCount - 1);
        if (curRange.commonAncestorContainer.nodeType == 3 && curRange.startOffset > 0) {
            // we are in child selection. The characters of the text node is being deleted
            return;
        }

        var range = document.createRange();
        if (selection.anchorNode != this) {
            // selection is in character mode. expand it to the whole editable field
            range.selectNodeContents(this);
            range.setEndBefore(selection.anchorNode);
        } else if (selection.anchorOffset > 0) {
            range.setEnd(this, selection.anchorOffset);
        } else {
            // reached the beginning of editable field
            return;
        }
        range.setStart(this, range.endOffset - 1);


        var previousNode = range.cloneContents().lastChild;
        if (previousNode && previousNode.contentEditable == 'false') {
            // this is some rich content, e.g. smile. We should help the user to delete it
            range.deleteContents();
            event.preventDefault();
        }
    }
});

<div id="editable" contenteditable="true">foo <span contenteditable="false">Text to delete</span> bar</div>