JavaScript SOAP Client

In this article we propose a solution based on AJAX that has a great advantage with respect to those commonly found in Internet: calls are made to the Web Services.
This permits:

1. On the server side we only have to expose a Web Service with the required methods (instead of generating dynamic pages incorporating data that are based on a custom syntax or on a generic XML)
2. On the client side we use the WSDL (Web Service Description Language) to automatically generate a JavaScript proxy class so as to allow using the Web Service return types - that is similar to what Visual Studio does when a Web Reference is added to the solution.

The following diagram shows the SOAP Client workflow for asynchronous calls:

The Client invokes the SOAPClient.invoke method using a JavaScript function and specifying the following:

• Web Service URL (pls note that many browsers do not allow cross-domain calls for security reasons)
• Web method name
• Web method parameter values
• Call mode (async = true, sync = false)
• CallBack method invoked upon response reception (optional for sync calls)

The SOAPClient.invoke method executes the following operations (numbers refer to the previous diagram)

1. It gets the WSDL and caches the description for future requests
2. It prepares and sends a SOAP (v. 1.1) request to the server (invoking method and parameter values)
3. It processes the server reply using the WSDL so as to build the corresponding JavaScript objects to be returned

If the call mode is async, the CallBack method is invoked, otherwise it returns the corresponding object

Implementation

After having exposed our idea about consuming a Web Service via JavaScript, we only have to analyze the code.

Let's start with the class for the definition of the parameters to be passed to the Web method:SOAPClientParameters

function SOAPClientParameters()
{
    var _pl = new Array();
    this.add = function(name, value)
    {
        _pl[name] = value;
        return this;
    }
    this.toXml = function()
    {
        var xml = "";
        for(var p in _pl)
            xml += "<" + p + ">" + SOAPClientParameters._serialize(_pl[p]) + "</" + p + ">";
        return xml;  
    }
}

The code simply consists of an internal dictionary (associative array) with parameter name (key) and related value; theadd method allows appending new parameters, while toXml method - by calling the recursive functionSOAPClientParameters._serialize - provides XML serialization for SOAP request (seeSOAPClient._sendSoapRequest):

SOAPClientParameters._serialize = function(o)
{
    var s = "";
    switch(typeof(o))
    {
        case "string":
            s += o.replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;"); break;
        case "number":
        case "boolean":
            s += o.toString(); break;
        case "object":
            // Date
            if(o.constructor.toString().indexOf("function Date()") > -1)
            {
                var year = o.getFullYear().toString();
                var month = (o.getMonth() + 1).toString(); month = (month.length == 1) ? "0" + month : month;
                var date = o.getDate().toString(); date = (date.length == 1) ? "0" + date : date;
                var hours = o.getHours().toString(); hours = (hours.length == 1) ? "0" + hours : hours;
                var minutes = o.getMinutes().toString(); minutes = (minutes.length == 1) ? "0" + minutes : minutes;
                var seconds = o.getSeconds().toString(); seconds = (seconds.length == 1) ? "0" + seconds : seconds;
                var milliseconds = o.getMilliseconds().toString();
                var tzminutes = Math.abs(o.getTimezoneOffset());
                var tzhours = 0;
                while(tzminutes >= 60)
                {
                    tzhours++;
                    tzminutes -= 60;
                }
                tzminutes = (tzminutes.toString().length == 1) ? "0" + tzminutes.toString() : tzminutes.toString();
                tzhours = (tzhours.toString().length == 1) ? "0" + tzhours.toString() : tzhours.toString();
                var timezone = ((o.getTimezoneOffset() < 0) ? "+" : "-") + tzhours + ":" + tzminutes;
                s += year + "-" + month + "-" + date + "T" + hours + ":" + minutes + ":" + seconds + "." + milliseconds + timezone;
            }
            // Array
            else if(o.constructor.toString().indexOf("function Array()") > -1)
            {
                for(var p in o)
                {
                    if(!isNaN(p)) // linear array
                    {
                        (/function\s+(\w*)\s*\(/ig).exec(o[p].constructor.toString());
                        var type = RegExp.$1;
                        switch(type)
                        {
                            case "":
                                type = typeof(o[p]);
                            case "String":
                                type = "string"; break;
                            case "Number":
                                type = "int"; break;
                            case "Boolean":
                                type = "bool"; break;
                            case "Date":
                                type = "DateTime"; break;
                        }
                        s += "<" + type + ">" + SOAPClientParameters._serialize(o[p]) + "</" + type + ">"
                    }
                    else // associative array
                        s += "<" + p + ">" + SOAPClientParameters._serialize(o[p]) + "</" + p + ">"
                }
            }
            // Object or custom function
            else
                for(var p in o)
                    s += "<" + p + ">" + SOAPClientParameters._serialize(o[p]) + "</" + p + ">";
            break;
        default:
            throw new Error(500, "SOAPClientParameters: type '" + typeof(o) + "' is not supported");
    }
    return s;
}

Let's define the SOAPClient class, which can only contain static methods in order to allow async calls, and the only "public" method within this class: SOAPClient.invoke

NOTE: since JavaScript does not foresee access modifiers - such as "public", "private", "protected", etc. - we'll use the "_" prefix to indicate private methods.

function SOAPClient() {}

SOAPClient.invoke = function(url, method, parameters, async, callback)
{
    if(async)
        SOAPClient._loadWsdl(url, method, parameters, async, callback);
    else
        return SOAPClient._loadWsdl(url, method, parameters, async, callback);
}

SOAPClient.invoke method interface is described above; our implementation checks whether the call is async (call result will be passed to the callback method) or sync (call result will be directly returned). The call to the Web Service begins by invoking SOAPClient._loadWsdl method:

SOAPClient._loadWsdl = function(url, method, parameters, async, callback)
{
    // load from cache?
    var wsdl = SOAPClient_cacheWsdl[url];
    if(wsdl + "" != "" && wsdl + "" != "undefined")
        return SOAPClient._sendSoapRequest(url, method, parameters, async, callback, wsdl);
    // get wsdl
    var xmlHttp = SOAPClient._getXmlHttp();
    xmlHttp.open("GET", url + "?wsdl", async);
    if(async)
    {
        xmlHttp.onreadystatechange = function()
        {
            if(xmlHttp.readyState == 4)
                SOAPClient._onLoadWsdl(url, method, parameters, async, callback, xmlHttp);
        }
    }
    xmlHttp.send(null);
    if (!async)
        return SOAPClient._onLoadWsdl(url, method, parameters, async, callback, xmlHttp);
}

The method searches the cache for the same WSDL in order to avoid repetitive calls:

SOAPClient_cacheWsdl = new Array();

If the WSDL is not found in the cache (it's the first call in current context) it is requested to the server by XMLHttpRequest, according the required mode (sync or not). Once an answer is obtained from the server, theSOAPClient._onLoadWsdl method is invoked:

SOAPClient._onLoadWsdl = function(url, method, parameters, async, callback, req)
{
    var wsdl = req.responseXML;
    SOAPClient_cacheWsdl[url] = wsdl;    // save a copy in cache
    return SOAPClient._sendSoapRequest(url, method, parameters, async, callback, wsdl);
}

A WSDL copy is stored into the cache and then the SOAPClient._sendSoapRequest method is executed:

SOAPClient._sendSoapRequest = function(url, method, parameters, async, callback, wsdl)
{
    // get namespace
    var ns = (wsdl.documentElement.attributes["targetNamespace"] + "" == "undefined") ? wsdl.documentElement.attributes.getNamedItem("targetNamespace").nodeValue : wsdl.documentElement.attributes["targetNamespace"].value;
    // build SOAP request
    var sr =
                "<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
                "<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/\">" +
                "<soap:Body>" +
                "<" + method + " xmlns=\"" + ns + "\">" +
                parameters.toXml() +
                "</" + method + "></soap:Body></soap:Envelope>";
    // send request
    var xmlHttp = SOAPClient._getXmlHttp();
    xmlHttp.open("POST", url, async);
    var soapaction = ((ns.lastIndexOf("/") != ns.length - 1) ? ns + "/" : ns) + method;
    xmlHttp.setRequestHeader("SOAPAction", soapaction);
    xmlHttp.setRequestHeader("Content-Type", "text/xml; charset=utf-8");
    if(async)
    {
        xmlHttp.onreadystatechange = function()
        {
            if(xmlHttp.readyState == 4)
                SOAPClient._onSendSoapRequest(method, async, callback, wsdl, xmlHttp);
        }
    }
    xmlHttp.send(sr);
    if (!async)
        return SOAPClient._onSendSoapRequest(method, async, callback, wsdl, xmlHttp);
}

The service namespace is taken out of the WSDL (using different XPath queries for Internet Explorer and Mozilla / FireFox), then a SOAP v. 1.1 request is created and submitted. The SOAPClient._onSendSoapRequest method will be invoked upon receiving the server response:

SOAPClient._onSendSoapRequest = function(method, async, callback, wsdl, req)
{
    var o = null;
    var nd = SOAPClient._getElementsByTagName(req.responseXML, method + "Result");
    if(nd.length == 0)
    {
        if(req.responseXML.getElementsByTagName("faultcode").length > 0)
        {
            if(async || callback)
                o = new Error(500, req.responseXML.getElementsByTagName("faultstring")[0].childNodes[0].nodeValue);
            else
                throw new Error(500, req.responseXML.getElementsByTagName("faultstring")[0].childNodes[0].nodeValue);          
        }
    }
    else
        o = SOAPClient._soapresult2object(nd[0], wsdl);
    if(callback)
        callback(o, req.responseXML);
    if(!async)
        return o;      
}

The server response is processed looking for faults: if found, an error is raised. Instead, if a correct result is obtained, a recursive function will generate the return type by using the service description:

SOAPClient._soapresult2object = function(node, wsdl)
{
    var wsdlTypes = SOAPClient._getTypesFromWsdl(wsdl);
    return SOAPClient._node2object(node, wsdlTypes);
}

SOAPClient._node2object = function(node, wsdlTypes)
{
    // null node
    if(node == null)
        return null;
    // text node
    if(node.nodeType == 3 || node.nodeType == 4)
        return SOAPClient._extractValue(node, wsdlTypes);
    // leaf node
    if (node.childNodes.length == 1 && (node.childNodes[0].nodeType == 3 || node.childNodes[0].nodeType == 4))
        return SOAPClient._node2object(node.childNodes[0], wsdlTypes);
    var isarray = SOAPClient._getTypeFromWsdl(node.nodeName, wsdlTypes).toLowerCase().indexOf("arrayof") != -1;
    // object node
    if(!isarray)
    {
        var obj = null;
        if(node.hasChildNodes())
            obj = new Object();
        for(var i = 0; i < node.childNodes.length; i++)
        {
            var p = SOAPClient._node2object(node.childNodes[i], wsdlTypes);
            obj[node.childNodes[i].nodeName] = p;
        }
        return obj;
    }
    // list node
    else
    {
        // create node ref
        var l = new Array();
        for(var i = 0; i < node.childNodes.length; i++)
            l[l.length] = SOAPClient._node2object(node.childNodes[i], wsdlTypes);
        return l;
    }
    return null;
}

SOAPClient._extractValue = function(node, wsdlTypes)
{
    var value = node.nodeValue;
    switch(SOAPClient._getTypeFromWsdl(node.parentNode.nodeName, wsdlTypes).toLowerCase())
    {
        default:
        case "s:string":          
            return (value != null) ? value + "" : "";
        case "s:boolean":
            return value + "" == "true";
        case "s:int":
        case "s:long":
            return (value != null) ? parseInt(value + "", 10) : 0;
        case "s:double":
            return (value != null) ? parseFloat(value + "") : 0;
        case "s:datetime":
            if(value == null)
                return null;
            else
            {
                value = value + "";
                value = value.substring(0, (value.lastIndexOf(".") == -1 ? value.length : value.lastIndexOf(".")));
                value = value.replace(/T/gi," ");
                value = value.replace(/-/gi,"/");
                var d = new Date();
                d.setTime(Date.parse(value));                                      
                return d;              
            }
    }
}

SOAPClient._getTypesFromWsdl = function(wsdl)
{
    var wsdlTypes = new Array();
    // IE
    var ell = wsdl.getElementsByTagName("s:element");  
    var useNamedItem = true;
    // MOZ
    if(ell.length == 0)
    {
        ell = wsdl.getElementsByTagName("element");   
        useNamedItem = false;
    }
    for(var i = 0; i < ell.length; i++)
    {
        if(useNamedItem)
        {
            if(ell[i].attributes.getNamedItem("name") != null && ell[i].attributes.getNamedItem("type") != null)
                wsdlTypes[ell[i].attributes.getNamedItem("name").nodeValue] = ell[i].attributes.getNamedItem("type").nodeValue;
        }  
        else
        {
            if(ell[i].attributes["name"] != null && ell[i].attributes["type"] != null)
                wsdlTypes[ell[i].attributes["name"].value] = ell[i].attributes["type"].value;
        }
    }
    return wsdlTypes;
}

SOAPClient._getTypeFromWsdl = function(elementname, wsdlTypes)
{
    var type = wsdlTypes[elementname] + "";
    return (type == "undefined") ? "" : type;
}

The SOAPClient._getElementsByTagName method optimizes XPath queries according to the available XML parser:

SOAPClient._getElementsByTagName = function(document, tagName)
{
    try
    {
        // trying to get node omitting any namespaces (latest versions of MSXML.XMLDocument)
        return document.selectNodes(".//*[local-name()=\""+ tagName +"\"]");
    }
    catch (ex) {}
    // old XML parser support
    return document.getElementsByTagName(tagName);
}

A factory function returns the XMLHttpRequest according to browser type:

SOAPClient._getXmlHttp = function()
{
    try
    {
        if(window.XMLHttpRequest)
        {
            var req = new XMLHttpRequest();
            // some versions of Moz do not support the readyState property and the onreadystate event so we patch it!
            if(req.readyState == null)
            {
                req.readyState = 1;
                req.addEventListener("load",
                                    function()
                                    {
                                        req.readyState = 4;
                                        if(typeof req.onreadystatechange == "function")
                                            req.onreadystatechange();
                                    },
                                    false);
            }
            return req;
        }
        if(window.ActiveXObject)
            return new ActiveXObject(SOAPClient._getXmlHttpProgID());
    }
    catch (ex) {}
    throw new Error("Your browser does not support XmlHttp objects");
}

SOAPClient._getXmlHttpProgID = function()
{
    if(SOAPClient._getXmlHttpProgID.progid)
        return SOAPClient._getXmlHttpProgID.progid;
    var progids = ["Msxml2.XMLHTTP.5.0", "Msxml2.XMLHTTP.4.0", "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLHTTP"];
    var o;
    for(var i = 0; i < progids.length; i++)
    {
        try
        {
            o = new ActiveXObject(progids[i]);
            return SOAPClient._getXmlHttpProgID.progid = progids[i];
        }
        catch (ex) {};
    }
    throw new Error("Could not find an installed XML parser");
}

Conclusions

By using a little (less than 13 KB) single JavaScript library and, on the server side, simply exposing a Web Service with remote methods, you can use AJAX to create dynamic Web applications with no need for reloading the entire page.