GObject.signal_connect_closure

function signal_connect_closure(instance: GObject.Object, detailed_signal: String, closure: Function, after: Boolean): Number(gulong) {
    // Gjs wrapper for g_signal_connect_closure()
}

Connects a closure to a signal for a particular object.

instance: the instance to connect to.
detailed_signal: a string of the form "signal-name::detail".
closure: the closure to connect.
after: whether the handler should be called before or after the default handler of the signal.
Returns: the handler id (always greater than 0 for successful connections)