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Asynchronous browser/server interaction

We will talk about a technology called AJAX (Asynchronous JavaScript and XML) which allows in some ways to improve the user interface behaviour with a good effect on the user experience.

You will need

  • Google Chrome (recommended, any browser with developer tools will do)
  • Sublime Text (recommended, any code editor will do... except Notepad)

Recommended reading

AJAX technology

From "classical" web sites (~ 1.0) to "desktop-like" web applications (~ 2.0)

  • Exchange data between browser and server

    • raw text
    • HTML
    • XML, JSON, ...
  • Using asynchronous techniques

    • Dynamic Script Loading
    • XMLHttpRequest
  • With JavaScript to orchestrate the communication

Also known as Single Page Application

Classic model

The page is fully reconstructed and reloaded to update the content (quite inefficient) and the user activity is interrupted, waiting the end of the action request

Asynchronous model

Only update a part of the content and the user activity is not interrupted (continue to play with UI while update is in progress)

The AJAX User eXperience is a key concept of the web as a platform

Starting file

This subject will use this index.html file as illustration. Be sure to download it, if you want to try and follow with the examples.

The basic idea of the following examples is about a user interface allowing to choose among a list of airports of Switzerland so as to get weather observation each time the user click on the button.

Dynamic Script Loading

At first we need to add some interaction with the button, calling a function named send, for example like this:

document.onreadystatechange = function () {
    if (document.readyState === "complete") {
        var btn = document.getElementById("get-observation");
        btn.addEventListener("click", send);

You can also decide to load the JavaScript at the end of the body ...

Dynamic Script Loading

Now it's time for a first asynchronous request like this:

function send() {
    var serviceURL = "";
    var code = document.getElementById("airportCode").selectedOptions[0].value;
    serviceURL+= "?icao=" + code;

    var DSLScript = document.createElement("script");
    DSLScript.src = serviceURL;
    DSLScript.type = "text/javascript";

    document.getElementById("status").childNodes[0].data = "Waiting ...";
  • we compose the web service URL call with a GET parameter
  • we insert a script element so as to trigger the loading request
  • given that Dynamic script loading is asynchronous, the code execution can go on, so we insert a status message in the UI

Web service, web resource, URL, HTTP GET, POST, ...

Let's already have a look at these slides:

Web resource getWeatherIcaoHTML

Let's click the button and nothing happens, except ...

  • we can see that a GET request in the background

  • we can see an error

ReferenceError: callback is not defined

  • we can see the result of the GET request (a JavaScript function call)
callback('<tr><td>Bern / Belp </td><td>16</td><td>29</td><td>clouds and visibility OK</td></tr>')

Something is missing!

We need to declare a callback function ...

Callback function declaration

This is the function which the loaded instruction does order to call

function callback(sMessage) {
    var info = document.getElementById("tableInfo");
    info.insertAdjacentHTML("beforeend", sMessage);

    document.getElementById("status").childNodes[0].data = "Done";
  • as soon as the script loaded is executed by the browser (because it was insert in a <script> element), it calls the function called callback
  • it comes with a value parameter which contains some HTML ready to be inserted in the page (it is a <tr> element)
  • insertAdjacentHTML is used to insert the new line in the table, at the right place (before end)

We get some ready to be used HTML which means the presentation logic is at the server-side. The data are already tailored!

First AJAX call ... clap clap clap!

Web resource getWeatherIcaoJSONP

Let's get only the data encoded with the JSON format.

Replace the service URL variable like this (and try it!)

var serviceURL = "";

Replace the callback function like this

function callback(sMessage) {
    var weatherInfo = JSON.parse(sMessage);

    document.getElementById("status").childNodes[0].data = "Done";
  • what we expect is a value parameter of the callback function which contains some JSON, so we parse it
  • AJAX with a response built of JSON data encapsulated in a function callback is called JSON with padding aka JSONP
  • so we get only data that we decide to display according to a dedicated function displayInfo ...

Create the HTML user interface

Add the following function to build the user interface so as to tailor the data we received

function displayInfo(info) {
    // clone the hidden row
    var newtr = document.getElementsByClassName("hidden").item(0).cloneNode(true);

    // feed the new row
    newtr.children[0].childNodes[0].data = info.stationName;
    newtr.children[1].childNodes[0].data = info.temperature;
    newtr.children[2].childNodes[0].data = info.humidity;
    newtr.children[3].childNodes[0].data = info.clouds;

    // append it to the table
    var tbody = document.getElementsByTagName("tbody").item(0);

We get only data which means the presentation logic is at the client-side. The client has to know how to tailor the data into the existing UI!

Web resource getWeatherIcaoJSON

Dynamic Script Loading is useful but less and less used. When possible, prefer the ultimate AJAX tool, the XMLHttpRequest object!

Replace the send function like this

function send() {
    var serviceURL = "";
    var code = document.getElementById("airportCode").selectedOptions[0].value;
    serviceURL+= "?icao=" + code;

    var xmlhttp = new XMLHttpRequest();"GET", serviceURL, true);
    xmlhttp.onreadystatechange = callback;

    document.getElementById("status").childNodes[0].data = "Waiting ...";

Everything with JavaScript now! AJAX request is configured using a dedicated XMLHttpRequest object (XHR).

May be you want to try the getWeatherIcaoJSON web service used here.

Callback function listener

The callback function is a listener we need to configure

Replace the callback function like this

function callback(evt) {
    console.log("State change => " + this.readyState);

    if (this.readyState === 4 && this.status === 200) {
        var weatherInfo = JSON.parse(this.responseText);

        document.getElementById("status").childNodes[0].data = "Done";
  • we parse the response as soon as the request is in state DONE and with a successful HTTP status,
  • it is stored as raw text in the responseText property of the XHR object
  • we have to parse it so as to extract the JavaScript object holding the four weather observations to display

Using jQuery to AJAX

Let's see how jQuery can help us to manage AJAX. So, we rewrite everything with jQuery. But, write less, do more!

At first, we restart with our clean starting index.html file and insert the library like this

<script src=""></script>

Then register a click event listener on the button

$(document).ready(function (e) {

Let's add an AJAX request:

  • there is the main method jQuery.ajax() or $.ajax() which offers all the possible functionalities.
  • there are also shorthand methods for the more common and specific types of AJAX requests with even less code - jQuery.get(), jQuery.getJSON(), jQuery.getScript(),, .load().

Use of $.ajax()

The send() function now looks like this

function send() {
        url: "",
        method: "GET",
        data: {
            icao: $("option:selected").val()
        dataType: "json"

    $("#status").text("Waiting ...");
  • $.ajax takes an object - it holds properties to configure the AJAX request
  • the service URL and HTTP method
  • the data object whose keys/values are used to complete the request
  • the expected data type of the result
  • we register the done function callback which is called when the request terminates successfully.

done callback function

Here is the callback function

function callbackOk(weatherInfo) {
    var arrayInfo = $.map(weatherInfo, function (ele) {
        return ele;

  • jQuery does the parsing for us because the request is set with dataType: "json"
  • so we receive the result straightforwardly as a JavaScript object
  • we use the $.map function to convert the object into an array ... why? see next ...

Display the result

Finally we need a display function in charge of updating the UI

function displayInfo(info) {
    var newtr = $(".hidden").clone();

    $(newtr).children().each(function (i) {

  • The info array is here useful to associate each value to a cell with a loop

The idea is to do post-processing of the result apart from the display

Now, let's make it fail!

We (I, in fact) remove this configuration from the server (see also Dis-E-nable CORS on Apache)

Header set Access-Control-Allow-Origin "*"

Just try now to get an observation ...

Nothing works anymore :-( ... and the console says something like

"Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at (Reason: CORS header ‘Access-Control-Allow-Origin’ missing)."

So, there is something to understand about:

  • SOP, which stands for Same Origin Policy
  • CORS, which stands for Cross-Origin Resource Sharing

Same-origin policy (SOP)

Since the mid of nineties:

  • important concept for web app security to protect access to the DOM
  • browser permits scripts contained in a first web page to access data in a second web page, but only if both web pages have the same origin
  • origin is the triple {protocol, host, port}
are of same origin (same protocol, host and port)
are not of same origin (different protocol, different host)
are not of same origin (different host)

Why does SOP protect the final user?

What if Same Origin Policy was not the default behaviour?

  • document.cookie is often used to authenticate sessions
  • given a final user visiting a banking website and does forget to log out
  • malicious JavaScript code running by another visited web page (e.g. in another tab) can do anything the user could do on the banking site
  • i.e. send requests to the banking site with the banking site's session cookie (e.g. get a list of transactions)

That would be really bad!

Same Origin Policy does apply on:

  • cookies
  • AJAX request (using XMLHTTPRequest object)
  • DOM access
  • data storage access (e.g. localStorage)

Relax it for AJAX ... using CORS

Sometimes the same-origin policy is too restrictive

  • many web applications require to interact with different origins through cross-origin requests
  • as soon as these origins are trusted, why would'nt it be possible?
  • by the way, the Dynamic Script Loading is not affected by the same-origin policy! So it is already possible!

Cross-Origin Resource Sharing (CORS)

  • recommended standard of the W3C to relax SOP
  • more secure than simply allowing all cross-origin requests (SOP is still the default behaviour)
  • browser/server interaction to decide if cross-origin request is safe or not
  • the server does decide to allow or not the cross-origin request

Simple CORS example

In context of a cross-origin request from a page loaded from server A :

  • browser sends to server B a HTTP OPTIONS request with the following header Origin: http://the.domain.of.server.A
  • server at http://the.different.domain.of.server.B may answer

Access-Control-Allow-Origin: *

  • means the cross-origin request is allowed, let's provide the data to the browser

So, let's rollback my server config ... and do something to handle a failure!

fail callback function

Beside the done callback, we register the fail function callback which is called when the request does fail.

  • Complete the $.ajax call like this
    // usual config options
  • And add the related function
function callbackFail(xhr) {
    $("#status").text("Failed :-(");

See also the jqXHR object returned by $.ajax

Now it's your turn ...

Use this crazy API



  • when loading the web application, a random fact is displayed on the top
  • when the search button is clicked the results of a free text search are piled up on the bottom according to the text entered by the user


You will find the final HTML file for this course here


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