javascriptを共有して、符号化と復号を行います.

    /**
    *htmlEncode  
    */
    function htmlEncode(str) {
        var s = "";
        if (str.length == 0) return "";
        s = str.replace(/&/g, "&");
        s = s.replace(/</g, "&lt;");
        s = s.replace(/>/g, "&gt;");
        s = s.replace(/\'/g, "&#39;");
        s = s.replace(/\"/g, "&quot;");
        return s;
    }
    /**
    *htmlDecode  
    */
    function htmlDecode(str) {
        var s = "";
        if (str.length == 0) return "";
        s = str.replace(/&amp;/g, "&");
        s = s.replace(/&lt;/g, "<");
        s = s.replace(/&gt;/g, ">");
        s = s.replace(/&#39;/g, "\'");
        s = s.replace(/&quot;/g, "\"");
        return s;
    }