xhttp.send();

Multiple Callback Functions

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) {
  const xhttp = new XMLHttpRequest();
  xhttp.onload = function() {cFunction(this);}
  xhttp.open("GET", url);
  xhttp.send();
}

function myFunction1(xhttp) {
  // action goes here
}
function myFunction2(xhttp) {
  // action goes here
}

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The onreadystatechange Property

The readyState property holds the status of the XMLHttpRequest.

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

The status property and the statusText properties hold 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() {
  const 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");
  xhttp.send();
}

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

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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)

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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).

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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.

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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.

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GET Requests

A simple GET request:

Example

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

n 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(); 

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();