Redo - undo javascript command is not working In IE browser while same working in chrome

I have craeted a HTML editor using asp.net and javscript . But redo and undo is not working in IE while same is working in chrome.Kindly suggest what i can do ?


<%----%> Bold Italic Underline
                    <img class="dvd" src="../../Images/tool_devid.gif" alt="" />

                    <a onclick="javascript:fontEdit('justifyleft')" title="Justify Left" class="tip" id="justifyleft">
                        <img src="../../Images/left.png" alt="" title="Justify Left" /></a>
                    <a onclick="javascript:fontEdit('justifycenter')" id="justifycenter" class="tip" title="Justify Center">
                        <img src="../../Images/center.png" alt="" /></a>
                    <a onclick="javascript:fontEdit('justifyright')" id="justifyright" title="Justify Right" class="tip">
                        <img src="../../Images/right.png" alt="" /></a>

                    <img class="dvd" src="../../Images/tool_devid.gif" alt="" />

                    <a onclick="javascript:fontEdit('insertorderedlist')" id="insertorderedlist" title="Order List" class="tip">
                        <img src="../../Images/numlist.png" alt="" /></a>
                    <a onclick="javascript:fontEdit('insertunorderedlist')" id="insertunorderedlist" title="Bullets List" class="tip">
                        <img src="../../Images/bullist.png" alt="" /></a>
                    <a onclick="javascript:fontEdit('outdent')" class="tip" id="outdent" title="Indent Less">
                        <img src="../../Images/outdent.png" alt="" /></a>
                    <a onclick="javascript:fontEdit('indent')" class="tip" id="indent" title="Indent More">
                        <img src="../../Images/indent.png" alt="" /></a>
                    <%--</a><a onclick="ChangeMode()">
			<img id="mode" src="images/mode.design.gif" alt="" title="Design View" style="height: 20px; width: 21px; border: none;" />--%>

                    <a onclick="javascript:fontEdit('undo')" class="tip" id="insertundo" title="Undo">
                        <img src="../../Images/undo.png" alt="undo" />
                    </a>

                    <a onclick="javascript:fontEdit('redo')" class="tip" title="Redo" id="insertredo">
                        <img src="../../Images/redo.png" alt="redo" />
                    </a>

                    <a onclick="var sLnk=prompt('Write the URL here','http:\/\/');if(sLnk&&sLnk!=''&&sLnk!='http://'){javascript:fontEdit('createlink',sLnk)} " class="tip" id="insertLink" title="Insert Link">
                        <img src="../../Images/Link.png" alt="hyperlink" />
                    </a>
                    <img class="dvd" src="../../Images/tool_devid.gif" alt="" />
                    <select id="fonts" class="select" onchange="fontEdit('fontname',this[this.selectedIndex].value)">
                        <option value="Arial">Font</option>
                        <option value="Antiqua">Antiqua</option>
                        <option value="Arial">Arial</option>
                        <option value="Blackletter">Blackletter</option>
                        <option value="Calibri">Calibri</option>
                        <option value="Courier New">Courier New</option>
                        <option value="Monotype Corsiva">Monotype</option>
                        <option value="Tahoma">Tahoma</option>
                        <option value="Times">Times</option>
                    </select>
                    <select id="size" class="select" onchange="fontEdit('fontsize',this[this.selectedIndex].value)">
                        <option value="0">Size</option>
                        <option value="1">1</option>
                        <option value="2">2</option>
                        <option value="3">3</option>
                        <option value="4">4</option>
                        <option value="5">5</option>
                    </select>
                    <select id="color" class="select" onchange="fontEdit('forecolor',this[this.selectedIndex].value)">
                        <option value="color">Color</option>
                        <option style="color: black;" value="black">Black</option>
                        <option style="color: red;" value="red">Red</option>
                        <option style="color: blue;" value="blue">Blue</option>
                        <option style="color: green;" value="green">Green</option>
                        <option style="color: #3b3b3b;" value="pink">Pink</option>
                    </select>

                    <a onclick="javascript:AddPageBreak1()" class="tip" title="pagebreak" id="pagebreak">
                        <img src="../../Images/pagebreak.jpg" alt="" />

                    </a>

                </div>

                <iframe id="textEditor" ondrop="drop(event);" ondragover="allowDrop(event);" class="txtArea">
                 
                </iframe>


            </div>
            <!--htmlEditor End -->

            <!--Letter Clients Start -->
            <div class="letterClient" id="divAllimagesContainer" style="display: none;">

                <!--Tabs area Start-->
                <ul class="tabMenu" id="letterTab">
                    <li><a href="#panel-1" data-toggle="tab">Logos</a></li>
                    <li><a href="#panel-2" data-toggle="tab">Signature</a></li>
                    <li><a href="#panel-3" data-toggle="tab">Bookmarks</a></li>
                </ul>
                <!--tab end -->
                <div class="clear"></div>

                <div class="tab-content">

                    <!--tab panel 1 -->
                    <div class="tab-pane" id="panel-1">
                        <div id="divImageLogo" style="display: block;"></div>
                    </div>

                    <!--tab panel 2 -->
                    <div class="tab-pane" id="panel-2">
                        <div id="divImageSig" style="display: block; margin-bottom: 20px;"></div>
                        <div id="divImageOthrs" style="display: block;"></div>
                    </div>

                    <!--tab panel 3 -->
                    <div class="tab-pane" id="panel-3">
                        <div id="dragList" contenteditable="false">
                            <ul id="divBookMarkContainer" contenteditable="false" class="bookMark"></ul>

                        </div>
                    </div>

                </div>
                <!--tab content end-->

            </div>
            <!--Letter Clients End -->

            <%--  <div id="textEditor" contenteditable="true" style="width: 800px; height: 400px; border: #777 solid 1px;"></div>--%>


            <div class="clear"></div>
        </div>

I function fontEdit(command, y) {

        switch (command) {
            case 'bold':

                if (document.getElementById('bold').onclick.toString().indexOf("over") != -1) {
                    document.getElementById('bold').src = "../../Images/bold.png";
                }
                else {
                    document.getElementById('bold').src = "../../Images/bold.png";
                    $("#bold").toggleClass("active");
                }
                break;
            case 'italic':
                if (document.getElementById('italic').onclick.toString().indexOf('over') != -1)
                    document.getElementById('italic').src = '../../Images/italic.png';
                else
                    document.getElementById('italic').src = '../../Images/italic.png';
                $("#italic").toggleClass("active");

                break;
            case 'underline':
                if (document.getElementById('underline').onclick.toString().indexOf('over') != -1)
                    document.getElementById('underline').src = '../../Images/under.png';
                else
                    document.getElementById('underline').src = '../../Images/under.png';
                $("#underline").toggleClass("active");

                break;
            case 'justifyleft':
                if (document.getElementById('justifyleft').onclick.toString().indexOf('over') != -1)
                    document.getElementById('justifyleft').src = '../../Images/left.png';
                else
                    document.getElementById('justifyleft').src = '../../Images/left.png';

                break;
            case 'justifycenter':
                if (document.getElementById('justifycenter').onclick.toString().indexOf('over') != -1)
                    document.getElementById('justifycenter').src = '../../Images/center.png';
                else
                    document.getElementById('justifycenter').src = '../../Images/center.png';

                break;
            case 'justifyright':
                if (document.getElementById('justifyright').onclick.toString().indexOf('over') != -1)
                    document.getElementById('justifyright').src = '../../Images/right.png';
                else
                    document.getElementById('justifyright').src = '../../Images/right.png';

                break;
            case 'insertorderedlist':
                if (document.getElementById('insertorderedlist').onclick.toString().indexOf('over') != -1)
                    document.getElementById('insertorderedlist').src = '../../Images/numlist.png';
                else
                    document.getElementById('insertorderedlist').src = '../../Images/numlist.png';
                $("#insertorderedlist").toggleClass("active");

                break;
            case 'insertunorderedlist':
                if (document.getElementById('insertunorderedlist').onclick.toString().indexOf('over') != -1)
                    document.getElementById('insertunorderedlist').src = '../../Images/bullist.png';
                else
                    document.getElementById('insertunorderedlist').src = '../../Images/bullist.png';
                $("#insertunorderedlist").toggleClass("active");

                break;
            case 'outdent':
                if (document.getElementById('outdent').onclick.toString().indexOf('over') != -1)
                    document.getElementById('outdent').src = '../../Images/outdent.png';
                else
                    document.getElementById('outdent').src = '../../Images/outdent.png';

                break;
            case 'indent':
                if (document.getElementById('indent').onclick.toString().indexOf('over') != -1)
                    document.getElementById('indent').src = '../../Images/indent.png';
                else
                    document.getElementById('indent').src = '../../Images/indent.png';

                break;
            case 'default':
                break;
        }


        //getIFrameDocument("textEditor").execCommand(command, "", y);
  
        document.getElementById("textEditor").contentWindow.focus();
        document.getElementById("textEditor").contentWindow.document.execCommand(command, "", y);
        document.getElementById("textEditor").contentWindow.focus();
     
    }