function A2U() {
//ASCII -> Unicode
if (parseFloat(ScriptEngineMajorVersion() + '.' + ScriptEngineMinorVersion()) < 5.5){
alert('您的脚本引擎版本过低，请升级为5.5以上');
return;
}
if (source.value == '') {
alert('文本框中没有代码！');
return;
}
result.value = '';
for (var i=0; i<source.value.length; i++)
result.value += '&#' + source.value.charCodeAt(i) + ';';
}
function U2A(val) {
//Unicode -> ASCII
//var code = source.value.match(/&#(\d+);/g);
var code = val.match(/&#(\d+);/g);
if (code == null) {
alert('文本框中没有合法的Unicode代码！');
return;
}
result = '';
for (var i=0; i<code.length; i++)
result += String.fromCharCode(code[i].replace(/[&#;]/g, ''));
return result;
}
function preview() {
var win = window.open();
win.document.open('text/html', 'replace');
win.document.writeln(result.value);
win.document.close();
}

<script type="text/javascript" src="Unicode.js"></script>
<script>
alert(U2A("世界?你好"));
</script>