Script代码中xhr.open("GET", URL, true)说open is not a function

Question

function ftn_button_clicked(){               xhr = new createXHR();                 //var url="\PointGenerate";                 xhr.open("GET", "PointGenerate.java", true);                           xhr.send();                 xhr.onreadystatechange = ajaxComplete;            }         function ajaxComplete(){             if(xhr.readyState === 4 && xhr.status === 200){                     document.getElementById("txt1").innerHTML.value = xhr.responseText;                     var num = xhr.responseText;                     window.alert("good");                     plotrandom(num);              }         }           function createXHR(){            if(window.XMLHttpRequest){                xhr = new XMLHttpRequest();                //window.alert("111"); check working or not            }            else{                xhr = new ActiveXObject("Microsoft.XMLHTTP");                //window.alert("1111");            }用游览器打开网页后，按按钮没反应， 就F12查看属性，游览器报错是这open那句话。 说is not a function. 

Answer 1

说明你没有正确初始化一个XMLHttpRequest对象

Answer 2

xhr没有定义吧