querySelector querySelectorAll 兼容处理，让ie支持querySelector 和 querySelectorAll

在简书上看到的这段兼容代码，看不太懂，先收藏起来了，欢迎大牛抛转解释

if (!document.querySelectorAll) {
        document.querySelectorAll = function (selectors) {
            var style = document.createElement('style'), elements = [], element;
            document.documentElement.firstChild.appendChild(style);
            document._qsa = [];
            style.styleSheet.cssText = selectors + '{x-qsa:expression(document._qsa && document._qsa.push(this))}';
            window.scrollBy(0, 0);
            style.parentNode.removeChild(style);
            while (document._qsa.length) {
                element = document._qsa.shift();
                element.style.removeAttribute('x-qsa');
                elements.push(element);
            }
            document._qsa = null;
            return elements;
        };
    }
    if (!document.querySelector) {
        document.querySelector = function (selectors) {
            var elements = document.querySelectorAll(selectors);
            return (elements.length) ? elements[0] : null;
        };
    }
    // 用于在IE6和IE7浏览器中，支持Element.querySelectorAll方法
    var qsaWorker = (function () {
        var idAllocator = 10000;
        function qsaWorkerShim(element, selector) {
            var needsID = element.id === "";
            if (needsID) {
                ++idAllocator;
                element.id = "__qsa" + idAllocator;
            }
            try {
                return document.querySelectorAll("#" + element.id + " " + selector);
            }
            finally {
                if (needsID) {
                    element.id = "";
                }
            }
        }
        function qsaWorkerWrap(element, selector) {
            return element.querySelectorAll(selector);
        }
        // Return the one this browser wants to use
        return document.createElement('div').querySelectorAll ? qsaWorkerWrap : qsaWorkerShim;
   })();

出处链接：https://www.jianshu.com/p/872d69047dda