Getting to know Ajax for the first time (1)

A preliminary understanding of Ajax and talk about their basic knowledge

1. Instantiate the XHR object

var  request=new XHLHttpRequest
Because of compatibility issues, it is generally written like this:
var request;
if (window.XHLHttpRequest){
        request=new XMLHttpRequest();//IE7+
    }else{
        request=newActiveXObject("Microsoft.XMLHttp");
        }

 2. HTTP request

1) The method or action of the HTTP request

1>get request: generally used for information acquisition

                   use url to pass parameters

                   There is also a limit to the number of messages sent, generally within 2000 characters

2>POST request: modify server resources

                        Unlimited number of messages sent

3. Response status code

The http status code consists of 3 digits, the first digit defines the status code type

1xx: Information class, indicating that a web browser request has been received and is being further processed

2xx: success

3xx: Redirect, indicating that the request was not successful and the client must take further action

4xx: Client error, indicating that the request submitted by the client is wrong

5xx: Server error, indicating that the server cannot complete the processing of the request

4. XMLHttpRequest gets the response

.responseText: Get response data of string type

.responseXML: Get response data in XML form

status and status Test: Return HTTP status codes as numbers and text

getAllReponseHeader(): Get all response headers

getResponseHeader(): The value of a field in the query response

5.readystate property

   0: The request is not initialized, OPEN has not been called

   1: The server link has been established, and OPEN has been called

   2: The request has been accepted, that is, the header information has been received

   3: The request is being processed, that is, the body of the response has been received

   4: The request is complete, the response is ready

6. Ajax GET request

     xmlhttp.open("GET","doem_get_asp",true);

     xmlhttp.send()

7. Ajax POST request

    xmlhttp.open("POST","dome_post_asp",true);

   xmlhttp.send()