Calling a Web Service from an HTML page using AJAX and HTTP POST (for all browsers)

Introduction

This article explains how you can call a Web Service using simple JavaScript and AJAX. This article discusses the client-side aspect. For the server-side aspect and source code, please visit this link.

Background

We can consume a Web Service from an HTML page using:

1. The webservice.htc approach for Internet Explorer.

Something like this: we include the above behaviour file like this:

<div id="handle" style=" behavior: url(webservice.htc);"></div>

and call the methods on the above div using:

handle.useService(SERVICE_URL + "?WSDL", "MyFunction");)

2. For Firefox:

var oSoapCall = new SOAPCall()

3. The universal approach where we make our own raw XML SOAP request and send this request, collect the response, and parse it like raw XML. Somewhat like this:

//create a soap request in xml
var soapRequest = "<soap:Envelope xmlns:xsi=\""
                 + "http://www.w3.org/2001/XMLSchema-instance\" "
                 + "xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" "
                 + "xmlns:soap=\" http://schemas.xmlsoap.org/soap/envelope/\">\n"
                 + "<soap:Body>\n"
                 + "<" + sMethod + "xmlns=\"" 
                 + SOAP_ACTION_BASE + "\">\n"
                 + "<op1>" + sOp1 + "</op1>\n"
                 + "<op2>" + sOp2 + "</op2>\n"
                 + "</" + sMethod + ">\n"
                 + "</soap:Body>\n"
                 + "</soap:Envelope>\n";
 //create a soap header
    var sSoapActionHeader = SOAP_ACTION_BASE + "/" + sMethod;
    crreat a request
  oXmlHttp.open("POST", SERVICE_URL, true);
  //when request is made, handle it appropriately
  oXmlHttp.onreadystatechange = function(){//call appropriate method};
  
  //set various headers for the request
  oXmlHttp.setRequestHeader("Content-Type", "text/xml");
  oXmlHttp.setRequestHeader("SOAPAction", sSoapActionHeader);
  //send the request
  oXmlHttp.send(sRequest);

Having said the above things, here's an example of how we can call the Web Service. This approach relates a little bit to the third approach above. However, for convenience, I have enabled HTTP communication for my Web Service here. This gives my Web Service the ability to communicate using HTTP POST and HTTP GET. For this, the following code should be added to the system.web section of the web.config:

<webServices>
  <protocols>
    <add name="HttpGet"/>
    <add name="HttpPost"/>
  </protocols>
</webServices>

The above step will enable GET and POST interaction with our Web Service class. This saves me the time to create an XML SOAP request like I created above and make the HTTP request instead of the SOAP request. Further, there are multiple workarounds and approaches for this as well. For a clue, please see the server-side article.

Using the Code

Once you do the above, fetching a response from the Web Service becomes very simple. Following is the code for fetching the response:

var xmlDoc;
var http =  new XMLHttpRequest();
var isFirefox = false;

function callGetLatestPoll()
{
    if(xmlDoc!=null)return;
    if (document.implementation && document.implementation.createDocument)
    {
        xmlDoc = document.implementation.createDocument("", "", null);
        xmlDoc.onload = fetchforfirefox;
    }
    else if (window.ActiveXObject)
    {
        xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
        xmlDoc.onreadystatechange = function () {
            if (xmlDoc.readyState == 4){isFirefox = false; fetchlatestpoll();}
        };
     }
    else
    {
        alert("Your browser do not support the script for polling service.");
        return;
    }
    xmlDoc.load("http://localhost/poll/PollService.asmx/GetLatestPoll");
}

function fetchlatestpoll()
{
    // code for reading and displaying data for internet explorer
    b1 = xmlDoc.documentElement;
    document.getElementById("txtId").innerText  = b1.childNodes.item(0).text;
    ..............
    .................
}

function fetchforfirefox()
{
    isFirefox=true;
    //code for reading and displaying data for firefox
    b1 = xmlDoc.childNodes.item("PollServiceBO");
    
    for(var temp1 = 0; temp1<b1.childNodes.length;temp1++)
    {
    if(b1.childNodes.item(temp1).childNodes.item(0)!=null)
    {
        var currentNode = b1.childNodes.item(temp1);
        switch(currentNode.nodeName)
        {
            case "ID":
              alert(currentNode.nodeName + " is Set");
              document.getElementById('txtId').innerHTML = 
                       currentNode.childNodes.item(0).nodeValue;
              break;
            .......
            .........
        }
    }

The above code is simple. My Web Service is PollService.asmx and my WebMethod is GetLatestPoll. It will return an object called PollServiceBO which looks like below:

<?xml version="1.0" encoding="utf-8" ?> 
- <PollServiceBO xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xmlns:xsd="http://www.w3.org/2001/XMLSchema" 
      xmlns="http://tempuri.org/">
  <ID>106</ID> 
  <Question>Test question1?</Question> 
  <option1>Test option1</option1> 
  <option2>Test option2</option2> 
  <option3>Test option3</option3> 
  <option4>Test option4.</option4> 
  <dateAdded>2008-07-17T16:39:04.49</dateAdded> 
  </PollServiceBO>

The real issue is how to post the data when a Web Service takes a parameter as input. Here is the code for it:

var xmlDoc;
var http =null;
var isFirefox = false;

function callPostMethod()
{
    var url = "http://localhost/poll/PollService.asmx/GetPollById";
    var params = "PollId=106";

    try
    {
        // Firefox, Opera 8.0+, Safari
        http =new XMLHttpRequest();
        isFirefox = true;
    }
    catch (e)
    {
        // Internet Explorer
        try
        {
            http =new ActiveXObject("Msxml2.XMLHTTP");
        }
        catch (e)
        {
            http =new ActiveXObject("Microsoft.XMLHTTP");
        }
    }

    http.open("POST", url, true);
    //Send the proper header information along with the request
    http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    http.setRequestHeader("Content-length", params.length);
    http.setRequestHeader("Connection", "close");
    http.onreadystatechange = function() {//Call a function when the state changes.
    if(http.readyState == 4 && http.status == 200) {
        xmlDoc = http.responseXML;
        if(isFirefox)
           {fetchforfirefox();}
        else
           {fetchlatestpoll();}
    }
}
http.send(params);
}

Yes, you got the point right! The above code calls the Web Service PollService.asmx's method GetPollById using HTTP POST ,method and the parameter passed to this service is PollId.

Note: For further discussion on the server-side aspects and source code, please view: HTML UI using Web Service.