- JavaScript callback functions tutorial: https://www.youtube.com/watch?v=pTbSfCT42_M&index=17&list=WL

**XMLHttpRequest** is an object to interact with servers. You can retrieve data from a URL without having to do a full page refresh. XMLHttpRequest is used heavily in Ajax programming - [MDN](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest).

So what is Ajax?

**Ajax** is a method of exchanging data with a server, and updating parts of a web page without reloading the entire page.

Let's dive into the code:

First, we need to make an instance from 'XMLHttpRequest' object.

var http = new XMLHttpRequest();

When we are doing a request it goes through 5 states:

* 0 : request is not initialized.

* 1 : request has been set up.

* 2 : request has been sent.

* 3 : request is in process.

* 4 : request is complete.

In the code below we are checking if the request is complete or not, and we check the status == 200 just to make sure that we do not get a 404 error - Read more about it here: [HTTP Status Code](https://httpstatuses.com).

http.onreadystatechange = function() {

if ( http.readyState == 4 && http.status == 200) {

console.log('Response from the server: ' + http.response);

}

}

There are methods to deal with a server like (GET, POST, UPDATE, DELETE…)

* GET: retrieve data from server.

* POST: send data to server.

* UPDATE: update data on the server.

* DELETE: delete date from the server.

To initialize a request we use [open](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/open) method. The syntax is:

XMLHttpRequest.open(method, url, async, user, password);

The parameters _method_ and _url_ are mandatory, _user_ and _password_ are optional. True is a default value for _async_.

http.open("GET", URL, true/false);

At the end we have to send our request to the server through [send](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/send) method. In our situation we are retrieving a data from the server, so we do not have to pass a parameter to the send request.

http.send();