AJAX - Server Response

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AJAX - Server Response
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The onreadystatechange Property
The readyState property holds the status of the XMLHttpRequest.

The onreadystatechange property defines a function to be executed when the readyState changes.

The status property and the statusText  property holds the status of the XMLHttpRequest object.

Property Description
onreadystatechange Defines a function to be called when the readyState property changes
readyState Holds the status of the XMLHttpRequest.
0: request not initialized
1: server connection established
2: request received
3: processing request
4: request finished and response is ready
status 200: "OK"
403: "Forbidden"
404: "Page not found"
For a complete list go to the Http Messages Reference
statusText Returns the status-text (e.g. "OK" or "Not Found")
The onreadystatechange function is called every time the readyState changes.

When readyState is 4 and status is 200, the response is ready:

Example
function loadDoc() {
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            document.getElementById("demo").innerHTML =
            this.responseText;
       }
    };
    xhttp.open("GET", "ajax_info.txt", true);
    xhttp.send();
}
»
The onreadystatechange event is triggered four times (1-4), one time for each change in the readyState.

Using a Callback Function
A callback function is a function passed as a parameter to another function.

If you have more than one AJAX task in a website, you should create one function for executing the XMLHttpRequest object, and one callback function for each AJAX task.

The function call should contain the URL and what function to call when the response is ready.

Example
loadDoc("url-1", myFunction1);

loadDoc("url-2", myFunction2);

function loadDoc(url, cFunction) {
  var xhttp;
  xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
      cFunction(this);
    }
  };
  xhttp.open("GET", url, true);
  xhttp.send();
}

function myFunction1(xhttp) {
  // action goes here
}
function myFunction2(xhttp) {
  // action goes here
}
»
Server Response Properties
Property Description
responseText get the response data as a string
responseXML get the response data as XML data
Server Response Methods
Method Description
getResponseHeader() Returns specific header information from the server resource
getAllResponseHeaders() Returns all the header information from the server resource
The responseText Property
The responseText property returns the server response as a JavaScript string, and you can use it accordingly:

Example
document.getElementById("demo").innerHTML = xhttp.responseText;
»
The responseXML Property
The XML HttpRequest object has an in-built XML parser.

The responseXML property returns the server response as an XML DOM object.

Using this property you can parse the response as an XML DOM object:

Example
Request the file cd_catalog.xml and parse the response:

xmlDoc = xhttp.responseXML;
txt = "";
x = xmlDoc.getElementsByTagName("ARTIST");
for (i = 0; i < x.length; i++) {
  txt += x[i].childNodes[0].nodeValue + "<br>";
  }
document.getElementById("demo").innerHTML = txt;
xhttp.open("GET", "cd_catalog.xml", true);
xhttp.send();
»
You will learn a lot more about XML DOM in the DOM chapters of this tutorial.

The getAllResponseHeaders() Method
The getAllResponseHeaders() method returns all header information from the server response.

Example
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    document.getElementById("demo").innerHTML =
    this.getAllResponseHeaders();
  }
};
»
The getResponseHeader() Method
The getResponseHeader() method returns specific header information from the server response.

Example
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    document.getElementById("demo").innerHTML =
    this.getResponseHeader("Last-Modified");
  }
};
xhttp.open("GET", "ajax_info.txt", true);
xhttp.send();
»

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