What is the best way to copy text to the clipboard (multi-browser)?

I have tried:

 function copyToClipboard(text) {
 if (window.clipboardData) { // Internet Explorer
 window.clipboardData.setData("Text", text);
 } else {
 unsafeWindow.netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
 const clipboardHelper = Components.classes["@mozilla.org/widget/clipboardhelper;1"].getService(Components.interfaces.nsIClipboardHelper);
 clipboardHelper.copyString(text);
 }
 }

But in Internet Explorer it gives a syntax error. In Firefox, it says *unsafeWindow is not defined*.

A nice trick without flashing: *https://stackoverflow.com/questions/17527870/how-does-trello-access-the-users-clipboard*