The XMLHttpRequest object is used to exchange data with a server.
To send a request to a server, we use the open() and send() methods of the XMLHttpRequest object:
| Method | Description |
|---|---|
| open(method,url,async) | Specifies the type of request, the URL, and if the request should be handled asynchronously or not. method: the type of request: GET or POST url: the location of the file on the server async: true (asynchronous) or false (synchronous) |
| send(string) | Sends the request off to the server. string: Only used for POST requests |
GET is simpler and faster than POST, and can be used in most cases.
However, always use POST requests when:
A simple GET request:
In the example above, you may get a cached result.
To avoid this, add a unique ID to the URL:
If you want to send information with the GET method, add the information to the URL:
A simple POST request:
To POST data like an HTML form, add an HTTP header with setRequestHeader(). Specify the data you want to send in the send() method:
| Method | Description |
|---|---|
| setRequestHeader(header,value) | Adds HTTP headers to the request. header: specifies the header name value: specifies the header value |
The url parameter of the open() method, is an address to a file on a server:
The file can be any kind of file, like .txt and .xml, or server scripting files like .asp and .php (which can perform actions on the server before sending the response back).
AJAX stands for Asynchronous JavaScript and XML, and for the XMLHttpRequest object to behave as AJAX, the async parameter of the open() method has to be set to true:
Sending asynchronous requests is a huge improvement for web developers. Many of the tasks performed on the server are very time consuming. Before AJAX, this operation could cause the application to hang or stop.
With AJAX, the JavaScript does not have to wait for the server response, but can instead:
When using async=true, specify a function to execute when the response is ready in the onreadystatechange event:
You will learn more about onreadystatechange in a later chapter.
To use async=false, change the third parameter in the open() method to false:
Using async=false is not recommended, but for a few small requests this can be ok.
Remember that the JavaScript will NOT continue to execute, until the server response is ready. If the server is busy or slow, the application will hang or stop.
Note: When you use async=false, do NOT write an onreadystatechange function - just put the code after the send() statement:
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