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lookupPrefix method

Browser support:

9

Retrieves the namespace prefix associated with the specified namespace URI in the scope of the current node.

Note: Internet Explorer supports the lookupPrefix method from version 9, but only for HTML documents, not for XML documents.

The reverse of this method is the lookupNamespaceURI method; it retrieves the namespace URI associated with the specified namespace prefix. If you want to know whether a namespace URI is the default namespace in the scope of a node, use the isDefaultNamespace method.

Syntax:

object.lookupPrefix (namespaceURI);

You can find the related objects in the Supported by objects section below.

Parameters:

namespaceURI

Required. String that specifies the namespace URI to search for.

Return value:

String that retrieves the matching namespace prefix. If no namespace prefix is associated with the specified namespace URI, it returns null.

Example HTML code 1:

This example illustrates the use of the lookupPrefix method:

Code

ajax.js

ns.xml

<head>
    <script type="text/javascript" src="ajax.js"></script>
    
    <script type="text/javascript">
        var httpRequest = null;
        
        function SendRequest () {
            if (!httpRequest) {
                httpRequest = CreateHTTPRequestObject ();   // defined in ajax.js
            }
            if (httpRequest) {          
                    // The requested file must be in the same domain that the page is served from.
                var url = "ns.xml";
                httpRequest.open ("GET", url, true);    // async
                httpRequest.onreadystatechange = OnStateChange;
                httpRequest.send (null);
            }
        }

        function OnStateChange () {
            if (httpRequest.readyState == 0 || httpRequest.readyState == 4) {
                if (IsRequestSuccessful (httpRequest)) {    // defined in ajax.js
                    GetNamespacePrefix ();
                }
                else {
                    alert ("Operation failed.");
                }
            }
        }

        function GetNamespacePrefix () {
            var xmlDoc = ParseHTTPResponse (httpRequest);   // defined in ajax.js
            if (!xmlDoc)
                return;
                
            var firstItem = xmlDoc.getElementsByTagName ("item")[0];
            if (firstItem.lookupPrefix) {
                var nsPrefix = firstItem.lookupPrefix ("http://help.dottoro.com/NS");
                alert ("The namespace prefix associated with the 'http://help.dottoro.com/NS' namespace URI: " + nsPrefix);
            }
            else {
                alert ("Your browser doesn't support the lookupPrefix method.");
            }
        }
    </script>
</head>
<body>
    <button onclick="SendRequest ()">Get the prefix associated with the 'http://help.dottoro.com/NS' namespace URI</button>
</body>


function CreateHTTPRequestObject () {
        // although IE supports the XMLHttpRequest object, but it does not work on local files.
    var forceActiveX = (window.ActiveXObject && location.protocol === "file:");
    if (window.XMLHttpRequest && !forceActiveX) {
        return new XMLHttpRequest();
    }
    else {
        try {
            return new ActiveXObject("Microsoft.XMLHTTP");
        } catch(e) {}
    }
    alert ("Your browser doesn't support XML handling!");
    return null;
}

function CreateMSXMLDocumentObject () {
    if (typeof (ActiveXObject) != "undefined") {
        var progIDs = [
                        "Msxml2.DOMDocument.6.0", 
                        "Msxml2.DOMDocument.5.0", 
                        "Msxml2.DOMDocument.4.0", 
                        "Msxml2.DOMDocument.3.0", 
                        "MSXML2.DOMDocument", 
                        "MSXML.DOMDocument"
                      ];
        for (var i = 0; i < progIDs.length; i++) {
            try { 
                return new ActiveXObject(progIDs[i]); 
            } catch(e) {};
        }
    }
    return null;
}

function CreateXMLDocumentObject (rootName) {
    if (!rootName) {
        rootName = "";
    }
    var xmlDoc = CreateMSXMLDocumentObject ();
    if (xmlDoc) {
        if (rootName) {
            var rootNode = xmlDoc.createElement (rootName);
            xmlDoc.appendChild (rootNode);
        }
    }
    else {
        if (document.implementation.createDocument) {
            xmlDoc = document.implementation.createDocument ("", rootName, null);
        }
    }
    
    return xmlDoc;
}

function ParseHTTPResponse (httpRequest) {
    var xmlDoc = httpRequest.responseXML;

            // if responseXML is not valid, try to create the XML document from the responseText property
    if (!xmlDoc || !xmlDoc.documentElement) {
        if (window.DOMParser) {
            var parser = new DOMParser();
            try {
                xmlDoc = parser.parseFromString (httpRequest.responseText, "text/xml");
            } catch (e) {
                alert ("XML parsing error");
                return null;
            };
        }
        else {
            xmlDoc = CreateMSXMLDocumentObject ();
            if (!xmlDoc) {
                return null;
            }
            xmlDoc.loadXML (httpRequest.responseText);

        }
    }

            // if there was an error while parsing the XML document
    var errorMsg = null;
    if (xmlDoc.parseError && xmlDoc.parseError.errorCode != 0) {
        errorMsg = "XML Parsing Error: " + xmlDoc.parseError.reason
                  + " at line " + xmlDoc.parseError.line
                  + " at position " + xmlDoc.parseError.linepos;
    }
    else {
        if (xmlDoc.documentElement) {
            if (xmlDoc.documentElement.nodeName == "parsererror") {
                errorMsg = xmlDoc.documentElement.childNodes[0].nodeValue;
            }
        }
    }
    if (errorMsg) {
        alert (errorMsg);
        return null;
    }

        // ok, the XML document is valid
    return xmlDoc;
}

    // returns whether the HTTP request was successful
function IsRequestSuccessful (httpRequest) {
        // IE: sometimes 1223 instead of 204
    var success = (httpRequest.status == 0 || 
        (httpRequest.status >= 200 && httpRequest.status < 300) || 
        httpRequest.status == 304 || httpRequest.status == 1223);
    
    return success;
}

<?xml version="1.0" encoding="utf-8"?>
<ns xmlns="http://help.dottoro.com/defaultNS" xmlns:dotto="http://help.dottoro.com/NS">
    <item dotto:color="red">
        <dotto:title>Namespace example</dotto:title>
        <dotto:description>Describes how to use namespaces with tags and attributes.</dotto:description>
        <dotto:pubDate>Wed, 17 Aug 2009 13:58:41 +0200</dotto:pubDate>
        <dotto:link>http://help.dottoro.com</dotto:link>
    </item>
    <order>
        <dotto:param name="sortBy" value="title" />
        <dotto:param name="sortDir" value="ascending" />
    </order>
</ns>