Example:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<textarea id="content"></textarea>
<input type="button" id="copyID" value="Copy" />
<script type="text/javascript">
var button = document.getElementById("copyID"),
input = document.getElementById("content");
button.addEventListener("click", function(event) {
event.preventDefault();
input.select();
document.execCommand("copy");
});
</script>
</body>
</html>document.execCommand("copy") copies the current selection to the clipboard