问对window.getSelection() api感到困惑

EN

var ele = $(".editor");
function pasteHtmlAtCaret(html) {
                    if(!ele.is(':focus')) {ele.focus();}
                    var sel, range;
                    if (window.getSelection) {
                        // IE9 and non-IE
                        sel = window.getSelection();
                        if (sel.getRangeAt && sel.rangeCount) {
                            range = sel.getRangeAt(0);
                            range.deleteContents();

                            // Range.createContextualFragment() would be useful here but is
                            // non-standard and not supported in all browsers (IE9, for one)
                            var el = document.createElement("div");
                            el.innerHTML = html;
                            var frag = document.createDocumentFragment(), node, lastNode;
                            while ((node = el.firstChild)) {
                                lastNode = frag.appendChild(node);
                            }
                            range.insertNode(frag);

                            // Preserve the selection
                            if (lastNode) {
                                range = range.cloneRange();
                                range.setStartAfter(lastNode);
                                range.collapse(true);
                                sel.removeAllRanges();
                                sel.addRange(range);
                            }
                        }
                    } else if (document.selection && document.selection.type !== "Control") {
                        // IE < 9
                        document.selection.createRange().pasteHTML(html);
                    }
                }