Ajax

1.兼容写法

        <script>
            //用没有定义的变量-保存
            //用没有定义的属性-undifind
        window.onload = function(){
            var oBtn = document.getElementById('btn1');
            
            oBtn.onclick = function(){    
                //兼容IE6
                var oAjax = window.XMLHttpRequest?new XMLHttpRequest():new ActiveXObject('Microsoft.XMLHTTP');
                
                oAjax.open('GET','https://cdn.bootcss.com/jquery/3.2.1/core.js',true);
                
                oAjax.send();
                oAjax.onreadystatechange = function()
                {
                    if(oAjax.readyState == 4)//通讯完成
                    {
                        if(oAjax.status==200)//通讯成功
                        {
                            alert(oAjax.responseText);    
                        }
                        else
                        {
                            alert('fail');    
                        }
                            
                            
                    }
                }
            }
        }
        </script>