<html>
<head>
<script>
function AddText(str) {
var ubb = document.getElementById("text1");
var ubbLength = ubb.value.length;
ubb.focus();
if (typeof document.selection != "undefined") {
document.selection.createRange().text = str;
}
else {
ubb.value = ubb.value.substr(0, ubb.selectionStart) + str + ubb.value.substring(ubb.selectionStart, ubbLength);
}
}
function Underline() {
var tUnderline = prompt("请输入要设置的下划线文字\n标签:[u][/u]", "")//
if (tUnderline == "" || tUnderline == null) {
return;
}
tUnderline = "[u]" + tUnderline + "[/u]"
AddText(tUnderline);
}
var lastFocusObj;
function setlastFocusObj(obj) {
lastFocusObj = obj;
}
</script>
</head>
<body>
<textarea id="text1" cols="40" rows="8"></textarea>
<a href="javascript:Underline();void(0)">下线线</a>
</body>
</html>
来源：https://www.cnblogs.com/059212315/archive/2011/06/03/2072125.html