The correct way checking the web browser notification already granted now?

What is the correct way to check the permission for the web browser notification? I am using notify.js from this https://github.com/alexgibson/notify.js/

I already follow the pattern from example, but i still get this error: "You must only call this before calling Notification.requestPermission(), otherwise this feature detect would trigger an actual notification!".

This is my method:

showNotification() {
        if (!Notify.needsPermission) {
            doNotification();
        } else if (Notify.isSupported()) {
            Notify.requestPermission(notifyStatus.onPermissionGranted, notifyStatus.onPermissionDenied);
        }
    }

And this is the method in notify.js for Notify.isSupported:

Notify.isSupported = function(perm) {
    if (!N || !N.requestPermission) {
        return false;
    }

    if (perm === 'granted' || N.permission === 'granted') {
        throw new Error('You must only call this before calling Notification.requestPermission(), otherwise this feature detect would trigger an actual notification!');
    }

    try {
        new N('');
    } catch (e) {
        if (e.name === 'TypeError') {
            return false;
        }
    }
    return true;
};

Please help, thanks.