[收藏]The XMLHttpRequest Object

作为Ajax三条腿之一的XMLHttpRequest是比较重要的工具，这里是w3shcools提供的比较官方的文档，供大家参考。比较重要的是最后的The XMLHttpRequest Object Reference，这个可以用来做手册用。

The XMLHttpRequest Object

The XMLHttpRequest object is supported in Internet Explorer 5.0+, Safari 1.2, Mozilla 1.0 / Firefox, and Netscape 7.

What is an HTTP Request?

With an HTTP request, a web page can make a request to, and get a response from a web server - without reloading the page. The user will stay on the same page, and he or she will not notice that scripts might request pages, or send data to a server in the background.

By using the XMLHttpRequest object, a web developer can change a page with data from the server after the page has loaded.

Google Suggest is using the XMLHttpRequest object to create a very dynamic web interface: When you start typing in Google's search box, a JavaScript sends the letters off to a server and the server returns a list of suggestions.

Is the XMLHttpRequest Object a W 3C Standard?

The XMLHttpRequest object is not a W 3C standard.

The W 3C DOM Level 3 "Load and Save" specification contains some similar functionality, but these are not implemented in any browsers yet. So, at the moment, if you need to send an HTTP request from a browser, you will have to use the XMLHttpRequest object.

Creating an XMLHttpRequest Object

For Mozilla, Firefox, Safari, and Netscape:

var xmlhttp=new XMLHttpRequest()

For Internet Explorer:

var xmlhttp=new ActiveXObject("Microsoft.XMLHTTP")

Example

<script type="text/javascript"> var xmlhttp function loadXMLDoc(url) { // code for Mozilla, etc. if (window.XMLHttpRequest)   {   xmlhttp=new XMLHttpRequest()   xmlhttp.onreadystatechange=xmlhttpChange   xmlhttp.open("GET",url,true)   xmlhttp.send(null)   } // code for IE else if (window.ActiveXObject)   {   xmlhttp=new ActiveXObject("Microsoft.XMLHTTP")     if (xmlhttp)     {     xmlhttp.onreadystatechange=xmlhttpChange     xmlhttp.open("GET",url,true)     xmlhttp.send()     }   } } function xmlhttpChange() { // if xmlhttp shows "loaded" if (xmlhttp.readyState==4)   {   // if "OK"   if (xmlhttp.status==200)     {     // ...some code here...     }   else     {     alert("Problem retrieving XML data")     }   } } </script>

Try it yourself using JavaScript

The syntax is a little bit different in VBScript: Try it yourself using VBScript

Note: An important property in the example above is the onreadystatechange property. This property is an event handler which is triggered each time the state of the request changes. The states run from 0 (uninitialized) to 4 (complete). By having the function xmlhttpChange() check for the state changing, we can tell when the process is complete and continue only if it has been successful.

Why are we Using async in our Examples?

All the examples here use the async mode (the third parameter of open() set to true).

The async parameter specifies whether the request should be handled asynchronously or not. True means that script continues to run after the send() method, without waiting for a response from the server. false means that the script waits for a response before continuing script processing. By setting this parameter to false, you run the risk of having your script hang if there is a network or server problem, or if the request is long (the UI locks while the request is being made) a user may even see the "Not Responding" message. It is safer to send asynchronously and design your code around the onreadystatechange event!

More Examples

Load a textfile into a div element with XML HTTP (JavaScript)

Make a HEAD request with XML HTTP (JavaScript)

Make a specified HEAD request with XML HTTP (JavaScript)

The XMLHttpRequest Object Reference

Methods

Method	Description
abort()	Cancels the current request
getAllResponseHeaders()	Returns the complete set of http headers as a string
getResponseHeader("headername")	Returns the value of the specified http header
open("method","URL",async,"uname","pswd")	Specifies the method, URL, and other optional attributes of a request The method parameter can have a value of "GET", "POST", or "PUT" (use "GET" when requesting data and use "POST" when sending data (especially if the length of the data is greater than 512 bytes. The URL parameter may be either a relative or complete URL. The async parameter specifies whether the request should be handled asynchronously or not. true means that script processing carries on after the send() method, without waiting for a response. false means that the script waits for a response before continuing script processing
send(content)	Sends the request
setRequestHeader("label","value")	Adds a label/value pair to the http header to be sent

Properties

Property	Description
onreadystatechange	An event handler for an event that fires at every state change
readyState	Returns the state of the object: 0 = uninitialized 1 = loading 2 = loaded 3 = interactive 4 = complete
responseText	Returns the response as a string
responseXML	Returns the response as XML. This property returns an XML document object, which can be examined and parsed using W 3C DOM node tree methods and properties
status	Returns the status as a number (e.g. 404 for "Not Found" or 200 for "OK")
statusText	Returns the status as a string (e.g. "Not Found" or "OK")

 

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