真诚的求一段 js 代码的讲解

想拿 HTML 直接生成 table of contents, 用 js 做于是找到了这个 http://stackoverflow.com/questions/187619/is-there-a-javascript-solution-to-generating-a-table-of-contents-for-a-page

原文如下

window.onload = function () {
    var toc = "";
    var level = 0;
    
    document.getElementById("contents").innerHTML =
        document.getElementById("contents").innerHTML.replace(
            /<h([\d])>([^<]+)<\/h([\d])>/gi,
            function (str, openLevel, titleText, closeLevel) {
                if (openLevel != closeLevel) {
                    return str;
                }

                if (openLevel > level) {
                    toc += (new Array(openLevel - level + 1)).join("<ul>");
                } else if (openLevel < level) {
                    toc += (new Array(level - openLevel + 1)).join("</ul>");
                }

                level = parseInt(openLevel);

                var anchor = titleText.replace(/ /g, "_");
                toc += "<li><a href=\"#" + anchor + "\">" + titleText
                    + "</a></li>";
                
                return "<h" + openLevel + "><a name=\"" + anchor + "\">"
                    + titleText + "</a></h" + closeLevel + ">";
            }
        );

    if (level) {
        toc += (new Array(level + 1)).join("</ul>");
    }

    document.getElementById("toc").innerHTML += toc;
};

Your page should be structured something like this:

<body>
    <div id="toc">
        <h3>Table of Contents</h3>
    </div>
    <hr/>
    <div id="contents">
        <h1>Fruits</h1>
        <h2>Red Fruits</h2>
        <h3>Apple</h3>
        <h3>Raspberry</h3>
        <h2>Orange Fruits</h2>
        <h3>Orange</h3>
        <h3>Tangerine</h3>
        <h1>Vegetables</h1>
        <h2>Vegetables Which Are Actually Fruits</h2>
        <h3>Tomato</h3>
        <h3>Eggplant</h3>
    </div>
</body>

You can see it in action at http://magnetiq.com/exports/toc.htm (Works in IE, FF, Safari, Opera)

主要就是document.getElementById("contents").innerHTML.replace(后面的看不懂， function (str, openLevel, titleText, closeLevel) {这里的几个变量怎么来的都没看懂....

希望有前辈可以帮忙讲解一下...