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)

        }else{

          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;

    }

  }