HackYourFuture · mkruijt · Oct 22, 2017 · Oct 21, 2017 · Oct 21, 2017 · Oct 22, 2017
diff --git a/Week5/REVIEW.MD b/Week5/REVIEW.MD
@@ -11,4 +11,53 @@ this review covers:
 
 ## Callbacks
 
-- JavaScript callback functions tutorial: https://www.youtube.com/watch?v=pTbSfCT42_M&index=17&list=WL
+- JavaScript callback functions tutorial: https://www.youtube.com/watch?v=pTbSfCT42_M&index=17&list=WL
+
+## XMLHttpRequest
+**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.
+```js
+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).
+```js
+ 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:
+```js
+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_.
+
+```js
+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.
+
+```js
+http.send();
+```