AJAX - XMLHttpRequest

The XMLHttpRequest object is used to request data from a server.

Send a Request To a Server

To send a request to a server, we use the open() and send() methods of the XMLHttpRequest object:

xhttp.open("GET", "ajax_info.txt", true);
xhttp.send();

Method	Description
open(method, url, async)	Specifies the type of request method: the type of request: GET or POST url: the server (file) location async: true (asynchronous) or false (synchronous)
send()	Sends the request to the server (used for GET)
send(string)	Sends the request to the server (used for POST)

The url - A File On a Server

The url parameter of the open() method, is an address to a file on a server:

xhttp.open("GET", "ajax_test.asp", true);

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).

Asynchronous - True or False?

Server requests should be sent asynchronously.

The async parameter of the open() method should be set to true:

xhttp.open("GET", "ajax_test.asp", true);

By sending asynchronously, the JavaScript does not have to wait for the server response, but can instead:

execute other scripts while waiting for server response
deal with the response after the response is ready

The default value for the async parameter is async = true.

You can safely remove the third parameter from your code.

Synchronous XMLHttpRequest (async = false) is not recommended because the JavaScript will stop executing until the server response is ready. If the server is busy or slow, the application will hang or stop.

GET or POST?

GET is simpler and faster than POST, and can be used in most cases.

However, always use POST requests when:

A cached file is not an option (update a file or database on the server).
Sending a large amount of data to the server (POST has no size limitations).
Sending user input (which can contain unknown characters), POST is more robust and secure than GET.

GET Requests

A simple GET request:

Example

xhttp.open("GET", "demo_get.asp");
xhttp.send();

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In the example above, you may get a cached result. To avoid this, add a unique ID to the URL:

Example

xhttp.open("GET", "demo_get.asp?t=" + Math.random());
xhttp.send();

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If you want to send information with the GET method, add the information to the URL:

Example

xhttp.open("GET", "demo_get2.asp?fname=Henry&lname=Ford");
xhttp.send();

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How the server uses the input and how the server responds to a request, is explained in a later chapter.

POST Requests

A simple POST request:

Example

xhttp.open("POST", "demo_post.asp");
xhttp.send();

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To POST data like an HTML form, add an HTTP header with setRequestHeader(). Specify the data you want to send in the send() method:

Example

xhttp.open("POST", "ajax_test.asp");
xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xhttp.send("fname=Henry&lname=Ford");

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Method	Description
setRequestHeader(header, value)	Adds HTTP headers to the request header: specifies the header name value: specifies the header value

Synchronous Request

To execute a synchronous request, change the third parameter in the open() method to false:

xhttp.open("GET", "ajax_info.txt", false);

Sometimes async = false are used for quick testing. You will also find synchronous requests in older JavaScript code.

Since the code will wait for server completion, there is no need for an onreadystatechange function:

Example

xhttp.open("GET", "ajax_info.txt", false);
xhttp.send();
document.getElementById("demo").innerHTML = xhttp.responseText;

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Modern developer tools are encouraged to warn about using synchronous requests and may throw an InvalidAccessError exception when it occurs.

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