Friday, December 13, 2013

XMLHttpRequest, REST and the Rich User Experience

Over the last few years Javascript has come of age. Gone are the inconsistent browser implementations, so not only is CSS mature enough to be used seriously, but so is Javascript. And now with the major browsers all supporting the XMLHttpRequest object, we can create a truely interactive interface to our web applications.
Using a combination of Javascript with the XMLHttpRequest object, a server side language like PHP and REST to do the talking in between, we can add dynamic elements to otherwise clumsy HTML forms.

Using the XMLHttpRequest Object

XMLHttpRequest is a Microsoft addition to IE5+ that was cloned by the Mozilla Project and now by Apple for Safari and Opera (at the time of writing support in Safari and Opera are new and incomplete, but this should improve with time). It allows Javascript to send a HTTP request, but unlike using an Image object, the XMLHttpRequest object allows you to receive and process the response, it will even parse out an XML response for you (thus the XML in XMLHttpRequest, although any response file type will do).
Creating and using the object is easy. Depending on whether the client is using IE with ActiveX or a standards compliant browser, there are different ways of instantiating the object, so this function wraps the process up into an easy to call package:
function getHTTPObject() {
    if (typeof XMLHttpRequest != 'undefined') {
        return new XMLHttpRequest();
    } try {
        return new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
        try {
            return new ActiveXObject("Microsoft.XMLHTTP");
        } catch (e) {}
    return false;
Note: This is an updated version of my original Javascript as written by Simon Willison.
Then it's just a case of setting the objects properties and letting it go:
var http = getHTTPObject();"GET", "", true);
http.onreadystatechange = function() {
    if (http.readyState == 4) {

And Now the REST

There are a couple of XML based RPC formats available and suitable for sending information to our Javascript (XML-RPC, SOAP), however they are all large, bloated and don't use HTTP as it was designed. REST on the other hand is small, simple and a proven technology, and perfect for linking our Javascript with our serverside functions.
  1. Since we're getting information from our server, we use our XMLHttpRequest object to issue a HTTP GET request to the URL of our serverside script.
  2. The script extracts request data from the URL querystring and processes the request responding with a plain text formatted file, or as a simple XML document.
  3. Our XMLHttpRequest object receives the response and parses out the data into Javascript variables. Our client has it's new data without the HTML page (the browser or the user) having to issue a separate HTTP request.
Obviously a new HTTP request has happened, but it has occured transparently in the background without our HTML page changing, allowing us to manipulate our pages DOM with the information received without having to do a full round trip to the server (and thus refreshing the page).

An Example

Time for an example. Lets say we have a HTML form that collects a users address, a usual form for an e-commerce system. Now to make our form more user friendly we'd like to allow the user to enter their postal code and get their address automatically entered into the form.
In the past this would have required a round trip to the server, reposting form data and refreshing the page (or using a popup window and some Javascript), but using the XMLHttpRequest object we can do it all transparently to the user.
So first we need our form, here's a quick XHTML file:

        Your details
address.html - HTML page markup
We've included our Javascript file address_from_postcode.js in the header and given our address fieldset a classname of address that we'll use to find and attach our behaviour to it's children. We do this rather than using an ID so that we can have multiple address fields on the same page.
window.onload = function() {
    var url = "\from\postcode.txt?postcode=";
    var fieldsets = document.getElementsByTagName("fieldset");
    for (var foo in fieldsets) {
        if (fieldsets[foo].className == "address") {
            var textareas = fieldsets[foo].getElementsByTagName("textarea");
            for (var bar in textareas) {
                if (textareas[bar].className == "address") {
                    fieldsets[foo].address = textareas[bar];
            if (fieldsets[foo].address) {
                var inputs = fieldsets[foo].getElementsByTagName("input");
                for (var bar in inputs) {
                    if (inputs[bar].className == "lookup") {
                        inputs[bar].http = getHTTPObject();
                        inputs[bar].working = false;
                        inputs[bar].onclick = lookupAddress;
                    if (inputs[bar].className == "postcode") {
                        fieldsets[foo].postcode = inputs[bar];
            } else {
                alert("No address textarea defined within address fieldset!");
    function lookupAddress() {
        if (!this.working) {
            var http = this.http;
            var address = this.parentNode.address;
  "GET", url + escape(this.parentNode.postcode.value), true);
            this.http.onreadystatechange = function() {
                if (http.readyState == 4) {
                    this.working = false;
                    address.innerHTML = http.responseText;
            this.working = true;
addressfrompostcode.js - Javascript behaviour layer
This code does the following:
  1. Search the DOM for all fieldsets with a class of "address".
  2. For each fieldset, get the first textarea with a class of "address".
  3. Also get the first inputs with a class of "lookup" and "postcode".
  4. Attach to the lookup button, a HTTP object and an on click event.
  5. On click of the button, make sure we're not already waiting for a HTTP response and if not issue the HTTP request.
  6. On reception of the HTTP response, set the address textarea to the recieved value.
So now to add this complex behaviour to our HTML forms, all we need to do is include the Javascript file and add the class names to the relevant elements.
Of course this is just a simple example, I've been using it to dynamically update dropdowns and autocomplete fields from existing data, the number of applications of this type of rich forms are unlimited.

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