Understanding of xmlHttp object methods and properties_Basic knowledge

1.1 我的理解：

1. 用户的每次操作，都会有数据产生。
2. 通过DOM或者JS编写对数据进行封装，或者浏览器自身对http协议的一些数据进行封装。
3. 通过xmlHttp对象的一些方法，传入数据参数，向http服务器发送请求。
4. 返回结果通过DOM进行处理。

2  xmlHttp对象的成员。

2.1 属性

1. onreadystatechange:当readyState属性值发生改变时，触发的事件处理句柄。

例子：xmlHttp.onreadystatechange = functionHandler;

   function functionHandler() {

if(xmlHttp.readyState == 4) {

alert("当readyState状态为4时，弹出此窗口！！！");

}

  }

//句柄只有方法名称，没有这对“()”括号。赋值时要注意理解。

2. readyState:这个属性表示状态；总共有五种状态：

// Because xmlHttp is written in a fixed way, so each step will be accompanied by a change of state, so the event handler is always monitored , execute the corresponding logic.

Code execution order:

var xmlHttpReq = new ActiveXObject("MSXML2.XMLHTTP.3.0");

xmlHttpReq.open("GET", "http://localhost/test.xml", false);

xmlHttpReq.send();

alert(xmlHttpReq.responseText);

2.2 Method

1. open(Method, Url, Syn, User, Password);

When you create a new xmlHttp object, you actually create a http request.

This method specifies the request method(GET/POST/PUT/PROPFIND), URL, asynchronous(The default is true), verification information.

adopts the asynchronous method (true), the callback function specified by the onreadystatechange attribute will be called when the state changes.

2. send();

The synchronous or asynchronous mode of this method depends on the Syn parameter in the open method, if Syn = = false, this method will wait for the request to complete or timeout before returning, if Syn == true, this method will return immediately.