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XML HttpRequest


All modern browsers have a built-in XMLHttpRequest object to request data from a server.


The XMLHttpRequest Object

The XMLHttpRequest object can be used to request data from a web server.

The XMLHttpRequest object is a developers dream, because you can:

  • Update a web page without reloading the page
  • Request data from a server - after the page has loaded
  • Receive data from a server  - after the page has loaded
  • Send data to a server - in the background

XMLHttpRequest Example

When you type a character in the input field below, an XMLHttpRequest is sent to the server, and some name suggestions are returned (from the server):

Example

Start typing a name in the input field below:

Name:   Suggestions:

The example above is explained in the AJAX chapters of this tutorial.


Sending an XMLHttpRequest

A common JavaScript syntax for using the XMLHttpRequest object looks much like this:

Example

var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
       // Typical action to be performed when the document is ready:
       document.getElementById("demo").innerHTML = xhttp.responseText;
    }
};
xhttp.open("GET", "filename", true);
xhttp.send();
来试试吧 »


Example Explained

The first line in the example above creates an XMLHttpRequest object:

var xhttp = new XMLHttpRequest();

The onreadystatechange property specifies a function to be executed every time the status of the XMLHttpRequest object changes:

xhttp.onreadystatechange = function()

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

if (this.readyState == 4 && this.status == 200)

The responseText property returns the server response as a text string.

The text string can be used to update a web page:

document.getElementById("demo").innerHTML = xhttp.responseText;

You will learn a lot more about the XMLHttpRequest object in the AJAX chapters of this tutorial.


Old Versions of Internet Explorer (IE5 and IE6)

Old versions of Internet Explorer (IE5 and IE6) do not support the XMLHttpRequest object.

To handle IE5 and IE6, check if the browser supports the XMLHttpRequest object, or else create an ActiveXObject:

Example

if (window.XMLHttpRequest) {
    // code for modern browsers
    xmlhttp = new XMLHttpRequest();
 } else {
    // code for old IE browsers
    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
来试试吧 »




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