Typeerror: Value Does Not Implement Interface Eventlistener

This works find in IE and Chrome, however, in Firefox, I get the error: 'TypeError: Value does not implement interface EventListener.' This code is posted by a php script, so I had

Solution 1:

if (…) {
    function …() {            
        …
    }
}

is invalid. Function declarations must be on the top level of function or program bodies, inside of blocks their behaviour is implementation-dependent. Move it outside the if-statement:

functionLogout() {            
    var just_logged_out = 1;
    window.location.href = "logout-redirect.php";
}

if (document.getElementById("Logout") != null) {
    varLogout_Link = document.getElementById("Logout");
    Logout_Link.addEventListener('click', Logout, true);
}

So if the function was not declared [correctly], what's that odd error message? You might've expected something like WRONG ARGUMENTS ERROR: handler is 'undefined'. But actually it's not, Logout does refer to your link element. And that does indeed not implement the EventListener interface (JavaScript functions do).