为XML原型函数的JavaScript定义原型属性

我正在使用此链接中提供的自定义javascript函数http://km0.la/js/mozXPath/ 在FireFox中实现特定的XML功能

代码如下：

// mozXPath
// Code licensed under Creative Commons Attribution-ShareAlike License 
// http://creativecommons.org/licenses/by-sa/2.5/
if( document.implementation.hasFeature("XPath", "3.0") ) {
    if( typeof XMLDocument == "undefined" ) { XMLDocument = Document; }
    XMLDocument.prototype.selectNodes = function(cXPathString, xNode) {
        if( !xNode ) { xNode = this; }
        var oNSResolver = this.createNSResolver(this.documentElement);
        var aItems = this.evaluate(cXPathString, xNode, oNSResolver, 
                     XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null);
        var aResult = [];
        for( var i = 0; i < aItems.snapshotLength; i++) {
            aResult[i] = aItems.snapshotItem(i);
        }
        return aResult;
    }
    XMLDocument.prototype.selectSingleNode = function(cXPathString, xNode) {
        if( !xNode ) { xNode = this; } 
        var xItems = this.selectNodes(cXPathString, xNode);
        if( xItems.length > 0 ){ return xItems[0]; }
        else{ return null; }
    }
    Element.prototype.selectNodes = function(cXPathString) {
        if(this.ownerDocument.selectNodes) { 
            return this.ownerDocument.selectNodes(cXPathString, this);
        }
        else { throw "For XML Elements Only"; }
    }
    Element.prototype.selectSingleNode = function(cXPathString) {   
        if(this.ownerDocument.selectSingleNode) {
            return this.ownerDocument.selectSingleNode(cXPathString, this);
        }
        else { throw "For XML Elements Only"; }
    }
}

我想做的是将属性nodeTypedValue添加到selectSingleNode函数中，但我不确定如何执行此操作。在Element.prototype.selectSingleNode函数中，我尝试添加：

this.prototype.nodeTypedValue = this.textContent;

但是，它给了我一个错误，说它是未定义的。我甚至尝试在函数之外添加它，只是为了简化它并获得概念，它还说它是未定义的：

var XMLObj.selectSingleNode.prototype.nodeTypedValue = XMLObj.textContent;
alert(XMLObj.selectSingleNode("cd_rank").nodeTypedValue);

---

基本上，我想我要做的是，向一个原型函数添加一个原型属性。但是我需要一些帮助。如何添加nodeTypedValue，以便编写XMLObj.selectSingleNodePath.nodeTypedValue？

好的，我想我已经知道如何在函数中添加它了，可能是因为运气而不是逻辑：

Element.prototype.selectSingleNode = function(cXPathString){    
    if(this.ownerDocument.selectSingleNode) {
        var result = this.ownerDocument.selectSingleNode(cXPathString, this);
        if (result != null) {
            result.nodeTypedValue = result.textContent;
        }
        return result;
    }
    else{throw "For XML Elements Only";}
}

Element.prototype.selectSingleNode = function(cXPathString){    
    if(this.ownerDocument.selectSingleNode) {
        var result = this.ownerDocument.selectSingleNode(cXPathString, this);
        if (result != null) {
            result.nodeTypedValue = result.textContent;
        }
        return result;
    }
    else{throw "For XML Elements Only";}
}