selectNodes与getElementsByTagName性能对比及跨浏览器兼容方案

selectNodes与getElementsByTagName性能对比及跨浏览器兼容方案

为了验证不同DOM查询方法的性能差异，进行了如下测试：

 
var stringToDom=function(text) {
var doc;
if(window.ActiveXObject) {
doc = new ActiveXObject("MSXML2.DOMDocument");
doc.loadXML(text).documentElement;
} else {
doc = (new DOMParser()).parseFromString(text,"text/xml");
} 
return doc;
} 
var xmlDoc=stringToDom("<body><a href='a'>a</a><a href='b'>b</a></body>"),
c,
d1=new Date();
for(var i=0;i<100000;i++){
c=xmlDoc.getElementsByTagName("a");
} 
document.write("getElementsByTagName: ",new Date()-d1);
d1=new Date();
try{
for(var i=0;i<100000;i++){
c=xmlDoc.selectNodes("a");
} 
document.write("<br/>selectNodes: ",new Date()-d1);
}catch(ex){document.write("<br/>error:"+ex)}

测试结果表明，在Internet Explorer浏览器中，selectNodes方法的执行速度明显优于getElementsByTagName。

然而，Firefox等标准浏览器并未原生支持selectNodes方法。通过查阅相关资料，可以使用XPathEvaluator来实现该功能。以下是具体的兼容实现代码，虽然功能可用，但其执行效率并不理想：

 
if (!window.ActiveXObject) {
(function(){
var oEvaluator=new XPathEvaluator(),oResult;
XMLDocument.prototype.selectNodes = function(sXPath) {
oResult = oEvaluator.evaluate(sXPath, this, null, XPathResult.ORDERED_NODE_ITERATOR_TYPE, null);
var aNodes = [];
if (oResult != null) {
var oElement = oResult.iterateNext();
while (oElement) {
aNodes[aNodes.length]=oElement;
oElement = oResult.iterateNext();
} 
} 
return aNodes;
} 
})() 
} 

evaluate(xpathExpression, contextNode, namespaceResolver, resultType, result);

Returns an XPathResult based on an XPath expression and other given parameters.

xpathExpression is a string representing the XPath to be evaluated.

contextNode specifies the context node for the query (see the [http://www.w3.org/TR/xpath XPath specification). It's common to pass document as the context node.

namespaceResolver is a function that will be passed any namespace prefixes and should return a string representing the namespace URI associated with that prefix. It will be used to resolve prefixes within the XPath itself, so that they can be matched with the document. null is common for HTML documents or when no namespace prefixes are used.

resultType is an integer that corresponds to the type of result XPathResult to return. Use named constant properties, such as XPathResult.ANY_TYPE, of the XPathResult constructor, which correspond to integers from 0 to 9.

result is an existing XPathResult to use for the results. null is the most common and will create a new XPathResult

完整的测试页面源码如下：

 
<!doctype HTML>
<html>
<head>
<title>selectNodes&getElementsByTagName</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="author" content="sohighthesky"/>
<meta name="Keywords" content="selectNodes vs getElementsByTagName"/>
</head>
<body>
</body>
<script type="text/javascript">
/* 
*author:sohighthesky -- http://www.cnblogs.com/sohighthesky
*content: selectNodes vs getElementsByTagName
*/ 
if (!window.ActiveXObject) {
(function(){
var oEvaluator=new XPathEvaluator(),oResult;
XMLDocument.prototype.selectNodes = function(sXPath) {
oResult = oEvaluator.evaluate(sXPath, this, null, XPathResult.ORDERED_NODE_ITERATOR_TYPE, null);
var aNodes = [];
if (oResult != null) {
var oElement = oResult.iterateNext();
while (oElement) {
aNodes[aNodes.length]=oElement;
oElement = oResult.iterateNext();
} 
} 
return aNodes;
} 
XMLDocument.prototype.selectSingleNode = function(sXPath) {
oResult = oEvaluator.evaluate(sXPath, this, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null);
// FIRST_ORDERED_NODE_TYPE returns the first match to the xpath.
return oResult==null?null:oResult.singleNodeValue;
} 
})() 
} 
var stringToDom=function(text) {
var doc;
if(window.ActiveXObject) {
doc = new ActiveXObject("MSXML2.DOMDocument");
doc.loadXML(text).documentElement;
} else {
doc = (new DOMParser()).parseFromString(text,"text/xml");
} 
return doc;
} 
var xmlDoc=stringToDom("<body><a href='a'>a</a><a href='b'>b</a></body>"),
c,
d1=new Date();
for(var i=0;i<100000;i++){
c=xmlDoc.getElementsByTagName("a");
} 
document.write("getElementsByTagName: ",new Date()-d1);
d1=new Date();
try{
for(var i=0;i<100000;i++){
c=xmlDoc.selectNodes("a");
} 
document.write("<br/>selectNodes: ",new Date()-d1);
}catch(ex){document.write("<br/>error:"+ex)}
/* 
var n=xmlDoc.selectSingleNode("body/a"),doc=xmlDoc.selectSingleNode("body");//alert(n.childNodes[0].nodeValue)
for(var i=0;i<10000;i++){
doc.appendChild(n.cloneNode(true))
} 
d1=new Date();
c=xmlDoc.getElementsByTagName("a");
document.write("<br/>getElementsByTagName: ",new Date()-d1);
d1=new Date();
c=xmlDoc.selectNodes("a");
document.write("<br/>selectNodes: ",new Date()-d1);
*/ 
</script>
</html>