getElement方法封装

   var get = {
        byId: function(id) {
            return document.getElementById(id)
        },
        byClass: function(sClass, oParent) {
            var aClass = [];
            var reClass = new RegExp("(^| )" + sClass + "( |$)");
            var aElem = this.byTagName("*", oParent);
            for (var i = 0; i < aElem.length; i++) reClass.test(aElem[i].className) && aClass.push(aElem[i]);
            return aClass
        },
        byTagName: function(elem, obj) {
            return (obj || document).getElementsByTagName(elem)
        }
    };