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 flash: How does Trello access the user's clipboard?