document.getElementsByClassName的理想实现

各种实现方式

Pure DOM

通常先使用getElementsByTagName("*")取出文档中所有元素，然后进行遍历，使用正则表达式找出匹配的元素放入一个数组返回。由于IE5不支持document.getElementsByTagName("*")，要使用分支document.all以防错误。

The Ultimate getElementsByClassName方案，作者为Robert Nyman，05年实现，可见老外许多东西在很早以前就走得很远了。

//三个参数都是必需的，查找一网页中5007个类名为“cell”的元素，IE8历时1828 ~ 1844毫秒，
//IE6为4610 ~ 6109毫秒，FF3.5为46 ~ 48毫秒，opera10为31 ~ 32毫秒，Chrome为23~ 26毫秒，
//safari4为19 ~ 20毫秒
function getElementsByClassName(oElm, strTagName, strClassName){
    var arrElements = (strTagName == "*" && oElm.all)? oElm.all :
        oElm.getElementsByTagName(strTagName);
    var arrReturnElements = new Array();
    strClassName = strClassName.replace(/\-/g, "\\-");
    var oRegExp = new RegExp("(^|\\s)" + strClassName + "(\\s|$)");
    var oElement;
    for(var i=0; i < arrElements.length; i++){
        oElement = arrElements[i];
        if(oRegExp.test(oElement.className)){
            arrReturnElements.push(oElement);
        }
    }
    return (arrReturnElements)
}

另一个实现，由Dustin Diaz（《JavaScript Design Patterns》的作者）提供，但兼容性不如上面的，不支持IE5。

//后两参数是可靠的，查找一网页中5007个类名为“cell”的元素，IE8历时78毫秒，IE6历时125~171毫秒
//FF3.5为42 ~ 48毫秒，opera10为31 毫秒，Chrome为22~ 25毫秒，safari4为18 ~ 19毫秒
var getElementsByClass = function(searchClass,node,tag) {
        var classElements = new Array();
        if ( node == null )
                node = document;
        if ( tag == null )
                tag = '*';
        var els = node.getElementsByTagName(tag);
        var elsLen = els.length;
        var pattern = new RegExp("(^|\\s)"+searchClass+"(\\s|$)");
        for (i = 0, j = 0; i < elsLen; i++) {
                if ( pattern.test(els[i].className) ) {
                        classElements[j] = els[i];
                        j++;
                }
        }
        return classElements;
}

还有个更古老级的，我从prototype.js1.01版本中找到的，它能支持多个类名的查找（上面两个不行）。它不支持IE5，效率一般般，但作为最早的框架之一，它已经做得很好，其他框架还没有想到这个呢！

//由于这是后期添加的，测试页面已被我删掉，没有做测试……
function getElementsByClassName(className, element) {
    var children = (element || document).getElementsByTagName('*');
    var elements = new Array();

    for (var i = 0; i < children.length; i++) {
        var child = children[i];
        var classNames = child.className.split(' ');
        for (var j = 0; j < classNames.length; j++) {
            if (classNames[j] == className) {
                elements.push(child);
                break;
            }
        }
    }

    return elements;
}

DOM Tree Walker

使用document.createTreeWalker，这是个比较不常用的二级DOM方法。可惜IE全系列歇菜。

//查找一网页中5007个类名为“cell”的元素，FF3.5为104 ~ 119毫秒，opera10为230 ~ 265毫秒，
//Chrome为119 ~ 128毫秒，safari为28 ~ 32毫秒
var getElementsByClassName = function(searchClass) {
    function acceptNode(node) {
        if (node.hasAttribute("class")) {
            var c = " " + node.className + " ";
            if (c.indexOf(" " + searchClass + " ") != -1)
                return NodeFilter.FILTER_ACCEPT;
        }
        return NodeFilter.FILTER_SKIP;
    }
    var treeWalker = document.createTreeWalker(document.documentElement,
        NodeFilter.SHOW_ELEMENT, acceptNode, true);
    var returnElements = [];
    if (treeWalker) {
        var node = treeWalker.nextNode();
        while (node) {
            returnElements.push(node);
            node = treeWalker.nextNode();
        }
    }
    return returnElements;
}

XPath

更加新式时髦的技术。

下面取自Prototype.js框架。

document.getElementsByClassName = function(className, parentElement) {
  if (Prototype.BrowserFeatures.XPath) {
    var q = ".//*[contains(concat(' ', @class, ' '), ' " + className + " ')]";
    return document._getElementsByXPath(q, parentElement);
  } else {
    var children = ($(parentElement) || document.body).getElementsByTagName('*');
    var elements = [], child;
    for (var i = 0, length = children.length; i < length; i++) {
      child = children[i];
      if (Element.hasClassName(child, className))
        elements.push(Element.extend(child));
    }
    return elements;
  }
};

由于这个是不能运行的，我们修改如下：

//查找一网页中5007个类名为“cell”的元素，FF3.5为33 ~ 48毫秒，opera10为31 ~ 32毫秒，
//Chrome为104 ~ 107毫秒，safari为18 ~ 21毫秒
var getElementsByClassName = function(searchClass,node,tag) {
    node = node || document;
    tag = tag || '*';
    var classes = searchClass.split(" "),
    patterns = "",
    xhtmlNamespace = "http://www.w3.org/1999/xhtml",
    namespaceResolver = (document.documentElement.namespaceURI === xhtmlNamespace)? xhtmlNamespace : null,
    returnElements = [],
    elements,
    _node;
    for(var j=0, jl=classes.length; j<jl; j+=1){
        patterns += "[contains(concat(' ', @class, ' '), ' " + classes[j] + " ')]";
    }
    try {
        elements = document.evaluate(".//" + tag + patterns, node, namespaceResolver, 0, null);
    }
    catch (e) {
        elements = document.evaluate(".//" + tag + patterns, node, null, 0, null);
    }
    while ((_node = elements.iterateNext()))  returnElements.push(_node);
    return returnElements;
}

当然如果游览器原生支持，就用原生的。

各主流游览器的支持情况
IE8	IE7	IE6	FF3	FF2	Saf3	Op9	Op10	Chrome
N	N	N	Y	N	Y	Y	Y	Y

速度比较一览

全平台兼顾速度的解决方案

由Robert Nyman提供。

var getElementsByClassName = function (className, tag, elm){
        if (document.getElementsByClassName) {
                getElementsByClassName = function (className, tag, elm) {
                        elm = elm || document;
                        var elements = elm.getElementsByClassName(className),
                                nodeName = (tag)? new RegExp("\\b" + tag + "\\b", "i") : null,
                                returnElements = [],
                                current;
                        for(var i=0, il=elements.length; i<il; i+=1){
                                current = elements[i];
                                if(!nodeName || nodeName.test(current.nodeName)) {
                                        returnElements.push(current);
                                }
                        }
                        return returnElements;
                };
        }
        else if (document.evaluate) {
                getElementsByClassName = function (className, tag, elm) {
                        tag = tag || "*";
                        elm = elm || document;
                        var classes = className.split(" "),
                                classesToCheck = "",
                                xhtmlNamespace = "http://www.w3.org/1999/xhtml",
                                namespaceResolver = (document.documentElement.namespaceURI === xhtmlNamespace)? xhtmlNamespace : null,
                                returnElements = [],
                                elements,
                                node;
                        for(var j=0, jl=classes.length; j<jl; j+=1){
                                classesToCheck += "[contains(concat(' ', @class, ' '), ' " + classes[j] + " ')]";
                        }
                        try     {
                                elements = document.evaluate(".//" + tag + classesToCheck, elm, namespaceResolver, 0, null);
                        }
                        catch (e) {
                                elements = document.evaluate(".//" + tag + classesToCheck, elm, null, 0, null);
                        }
                        while ((node = elements.iterateNext())) {
                                returnElements.push(node);
                        }
                        return returnElements;
                };
        }
        else {
                getElementsByClassName = function (className, tag, elm) {
                        tag = tag || "*";
                        elm = elm || document;
                        var classes = className.split(" "),
                                classesToCheck = [],
                                elements = (tag === "*" && elm.all)? elm.all : elm.getElementsByTagName(tag),
                                current,
                                returnElements = [],
                                match;
                        for(var k=0, kl=classes.length; k<kl; k+=1){
                                classesToCheck.push(new RegExp("(^|\\s)" + classes[k] + "(\\s|$)"));
                        }
                        for(var l=0, ll=elements.length; l<ll; l+=1){
                                current = elements[l];
                                match = false;
                                for(var m=0, ml=classesToCheck.length; m<ml; m+=1){
                                        match = classesToCheck[m].test(current.className);
                                        if (!match) {
                                                break;
                                        }
                                }
                                if (match) {
                                        returnElements.push(current);
                                }
                        }
                        return returnElements;
                };
        }
        return getElementsByClassName(className, tag, elm);
};
    

综合以上方案，我得出了一个最理想的实现——兼容IE5，让后面两个参数是可选的，能原生的原生，利用字面量与倒序循环提高效率……

//查找一网页中5007个类名为“cell”的元素，IE8历时1828 ~ 1844毫秒，
//IE6为125 ~ 172毫秒，IE8为93 ~ 94毫秒，FF3.5为0~1毫秒，opera10为0毫秒，Chrome为1毫秒，
//safari4为0毫秒
var getElementsByClassName = function(searchClass,node,tag) {
    if(document.getElementsByClassName){
        return  document.getElementsByClassName(searchClass)
    }else{    
        node = node || document;
        tag = tag || '*';
        var returnElements = []
        var els =  (tag === "*" && node.all)? node.all : node.getElementsByTagName(tag);
        var i = els.length;
        searchClass = searchClass.replace(/\-/g, "\\-");
        var pattern = new RegExp("(^|\\s)"+searchClass+"(\\s|$)");
        while(--i >= 0){
            if (pattern.test(els[i].className) ) {
                returnElements.push(els[i]);
            }
        }
        return returnElements;
    }
}

用法：

var collections = getElementsByClassName("red");

但它还是不如原生的getElementsByClassName，不能同时检索多个class

<h2 class="red cell title">安装支持</h2>

<span class="cell red  ">jjj</span>

<div class="filament_table  red cell">这是DIV</div>

#利用 var dd = getElementsByClassName("cell red") ，这三个元素都应该能被检索到！

因此，最最终的方案为：

      var getElementsByClassName = function (searchClass, node,tag) {
        if(document.getElementsByClassName){
          var nodes =  (node || document).getElementsByClassName(searchClass),result = [];
            for(var i=0 ;node = nodes[i++];){
              if(tag !== "*" && node.tagName === tag.toUpperCase()){
                result.push(node)
              }
            }
            return result
          }else{
            node = node || document;
            tag = tag || "*";
            var classes = searchClass.split(" "),
            elements = (tag === "*" && node.all)? node.all : node.getElementsByTagName(tag),
            patterns = [],
            current,
            match;
            var i = classes.length;
            while(--i >= 0){
              patterns.push(new RegExp("(^|\\s)" + classes[i] + "(\\s|$)"));
            }
            var j = elements.length;
            while(--j >= 0){
              current = elements[j];
              match = false;
              for(var k=0, kl=patterns.length; k<kl; k++){
                match = patterns[k].test(current.className);
                if (!match)  break;
              }
              if (match)  result.push(current);
            }
            return result;
          }
        }