What is an ”async page”? I guess you’re referring to a server component.

First of all, you need to do it in client components.

There’s a native property that can indicate the device of user: navigator.userAgent
However, it’s better to determine it with viewport width, since that user agent isn’t reliable

And it depends on what you are trying to do. Like detecting a touch screen device is much easier. “Mobile device” is something abstract and hard to be determined.

It seems that it is not possible to use the navigator on a server component page.

you can only access navigator in a client component. if you want to access the user agent on the server-side, you can use the [User-Agent](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent) header though as fuma said it's unreliable

Yep, as mentioned above, you must do it in a client component