How to call asynchronously web servers with Javascript ?

Problem :

We wanted to display or hide information from more than five web servers when loading a homepage. Instead of waiting until all the servers reply to us ( synchronous method) the customer wanted to see the information as soon as the server would reply. This solution presents a better user experience because the webpage is more responsive.


I will decompose the solution in several parts and explain each steps

  • A HTML/W3.CSS webpage with a button calling the javascript method
  • The Javascript code calling multiple web servers
  • The Javascript class that call a web server
  • The callback method which handle the response of the web server
  • The Javascript function which updates the webpage after obtaining a response from the server.

HTML web page

The html webpage contains a button and a text input. The text input is sent to the web servers when we click the button.

This is a simple example of webpage

The link to my javascript file :

<script type="text/javascript" src="./webserver.js">
<script type="text/javascript" src="./request.js">

When we click on the button we call a javascript function which will use the text content of the input “myinput”.

 <input name="myinput" id="myinput" class="w3-input w3-border w3-light-grey" type="text">
<button class="w3-btn w3-blue-grey" onclick="callWeb()">Load Web Servers</button>

callWeb() is a Javascript function which will retrieve the input value and call web servers.

function callWeb() {
    var input = document.getElementById('myinput'),
       myinput = input.value;
    if (myinput) {
    } else {
       alert('Please enter an input!');

To avoid the error “The character encoding of the HTML document was not declared” : I have added these lines in the HTML :

<meta content="text/html;charset=utf-8" http-equiv="Content-Type">
<meta content="utf-8" http-equiv="encoding">

For information in this example I use W3.CSS for the style. It is a modern CSS framework with built-in responsiveness. It is an equivalent of Bootstrap. More information at

Javascript : Calling multiple web servers

I will present the javascript function which send the request and the callback function which handle the response .

function loadWebServers(input)
    var value1 = input
    var value2 = ''
    var value3 = ''

In this example we call only two web servers. loadWebABC call a web service with XML request. loadWebX call another web service with a JSON request.

function loadWebABC(value1)
MyClassRequest.sendXmlRequest("", "myinput=”+ value1, "callbackWebABC");

When the server replies, the callback will handle the response :

function callbackWebABC(xml)
    var rootInfo = xml.getElementsByTagName("root-info");
    //parse more information

Javascript : Detail of the class that call the web server

XmlHttpRequest is the class used to call distant web servers.

If you use XMLHttpRequest from an extension, you should use it asynchronously. In this case, you receive a callback when the data has been received, which lets the browser continue to work as normal while your request is being handled.

MyClassRequest is the equivalent of a class in Javascript world. It is defined in a file called request.js :

var MyClassRequest = {
	loading : false,

updaters : new Array(),

	Updater : function(request, backFunction, param, isXml, isJson) {
		this.request = request;
		this.backFunction = backFunction;
		this.param = param;
		this.isXml = isXml;
		this.cancelled = false;
		this.isJson = isJson;

	createRequest : function() {
		var request = false;
		try {
			request = new XMLHttpRequest();
		} catch (trymicrosoft) {
			try {
				request = new ActiveXObject("Msxml2.XMLHTTP");
			} catch (othermicrosoft) {
				try {
					request = new ActiveXObject("Microsoft.XMLHTTP");
				} catch (failed) {
					request = false;
		return request;

	sendXmlRequest : function(url, urlParam, backFunction, param) {
		return this
				.sendRequest(url, urlParam, backFunction, param, true, false);

	sendRequest : function(url, urlParam, backFunction, param, isXml, isJson, timeout) {
		 if (typeof(timeout)==='undefined') {
			 timeout = 0;
		var request = this.createRequest();

		if (!request) {
			alert("Request not supported");
			return -1;
		} else {
			var index = this.updaters.length;
			this.updaters[index] = new MyClassRequest.Updater(request,
					backFunction, param, isXml, isJson);"POST", url, true);
			request.onreadystatechange = MyClassRequest.receiveRequest;
			if (timeout != null && timeout > 0) {
				request.timeout = timeout;
				request.ontimeout = MyClassRequest.timeoutRequest;
					"application/x-www-form-urlencoded; charset=UTF-8");

			return index;

As you can see the function sendXmlRequest is a sub function of sendRequest. sendRequest will use the object XMLHttpRequest or an equivalent for microsoft supported browser. This important part in this function is :

request.onreadystatechange = MyClassRequest.receiveRequest;

It is receiveRequest function which will trigger the callback function defined previously.

receiveRequest : function() {
		var stillLoading = false;
		for ( var i = MyClassRequest.updaters.length - 1; i >= 0; i--) {
			var updater = MyClassRequest.updaters[i];
			if (updater != null) {
				if (updater.request.readyState == 4) {
					MyClassRequest.updaters[i] = null;
					if (updater.cancelled == false) {
						if (!updater.request.status == 200) {
							alert("No response from server");
						} else {
                                                        if (updater.backFunction) {
				} else {
					stillLoading = true;

callbackFunction is the function which will actually call our callback function when we receive correctly a response form the distant web server.

function callbackFunction(updater)

var func = new Function("response", "param",
					+ "(response, param)");
	if (updater.isXml) {
		var xml = updater.request.responseXML;
		if (MyClassRequest.checkErrors(xml)) {
			func(false, updater.param);
		} else {
			func(xml, updater.param);
	} else {


Javascript : update the webpage

In the previous callback method we use a function showOrDisplayInfo(). This function will display or hide element in the web page. This is a simplified version of the function but as you can see it modifies an HTML element to show or hide this element.

function showOrDisplayInfo(infoextracted) {
for (var infoin infoextracted)
      var idElementToShow ="elementX"+ infoin;
         show( idElementToShow );

function show(element)
  if (element)

function hide(element)
  if (element)

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