BluetoothDevice - Web APIs
gatt; readonly attribute frozenarray uuids; promise watchadvertisements(); void unwatchadvertisements(); readonly attribute boolean watchingadvertisements; }; bluetoothdevice implements eventtarget; bluetoothdevice implements bluetoothdeviceeventhandlers; bluetoothdevice implements characteristiceventhandlers; bluetoothdevice implements serviceeventhandlers; properties bluetoothdevice.id read only a domstring that uniquely identifies a device.
... bluetoothdevice.watchingadvertisements read only if advertisments were activated using bluetoothdevice.watchadvertisements().

Bluetooth.getDevices() - Web APIs
the program can detect when a device comes online or into range by watching for bluetooth advertisements by calling bluetoothdevice.watchadvertisements() on that device.