2.选择器方法封装

var get = {

    byId: function(id) {

        return typeof id === "string" ? document.getElementById(id) : 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)

    }
}

调用方法
get.byId()  get.byClass()  get.byTageName()