Looks great, but it seems like positioning it as a beautiful flash message plugin rather than an alert replacement might be more apt.
Hopefully very few JS developers are using alerts in their production code (or dev for that matter!), and if they are they're probably not interested enough in user-friendly design to seek out this replacement.
I notice that even large apps like gmail and google calendar use native alerts (and I've been using them a bit more under certain circumstances—something has gone terribly wrong). What about native alerts in all situations equals unfriendly design—esp. when trying to mimic mobile native apps, but needing to span the gap from OS to OS?
I agree with their use under very particular circumstances, like in your example or (at least in desktop Chrome) for bringing attention to a window that isn't visible. I was referring more to using alerts as they're being used in the SweetAlerts examples (success messages and other "flash messages"). In these cases, UI-thread blocking seems bad, no?
I actually think SweetAlerts looks pretty nice for those kind of in-app notifications, which was the point I was trying to make. (Though seeing comments from people who have dug in further it seems like it has some issues)
Hopefully very few JS developers are using alerts in their production code (or dev for that matter!), and if they are they're probably not interested enough in user-friendly design to seek out this replacement.