<script>
    var code = '<table width="100"><tr><td>我爱你</td></tr></table>'.replace(/(\<)/g, " &lt; ");
    code = code.replace(/(\>)/g, "&gt; ");
    code = code.replace(/(\")/g, "&quot; ");
    alert(code);
</script>

------解决方案--------------------

JScript code
function ToHtmlEnCode(str)  
{
var s = "";
if (str.length == 0) return "";
s = str.replace(/&/g, "&amp;");
s = s.replace(/</g, "&lt;");
s = s.replace(/>/g, "&gt;");
s = s.replace(/ /g, "&nbsp;");
s = s.replace(/\'/g, "'");
s = s.replace(/\"/g, "&quot;");
s = s.replace(/\n/g, "<br>");
return s;
}

 function ToHTMLDeCode(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(/&nbsp;/g, " ");
s = s.replace(/'/g, "\'");
s = s.replace(/&quot;/g, "\"");
s = s.replace(/<br>/g, "\n");
return s;
}

var html = '<table width="100">&<tr><td>我爱你</td></tr></table>'
html = ToHtmlEnCode(html)
alert(html)
html = ToHTMLDeCode(html)
alert(html)