<html>
<head>
</head>
<body>
<textarea id="content" style="height: 50px">
foo
bar
baz
</textarea>
<button id="button">
clear
</button>
<script>
button.onclick = function(){
content.value = content.value.replace(/\r?\n/g, "")
}
</script>
</body>
</html>