How do I copy to the clipboard in JavaScript?

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 using Flash: How does Trello access the user’s clipboard?