The ShowDevices function shows the user a dialog box that contains a list of NXT bricks which are paired with the phone. The user can then select the brick to connect to via Bluetooth.

nxt.ShowDevices();

function OnStart()
{
  nxt = app.CreateNxt();
  app.SetMenu( "Connect" );
}
 
function OnMenu( name )
{
  if( name=="Connect" )
    nxt.ShowDevices();
}

NxtConnect.js

var nxt, lay;
function OnStart()
{
  lay = app.CreateLayout( "Linear", "VCenter, FillXY" );
 
  //Create Connect button.
  btnConnect = app.CreateButton( "[fa-plug] Connect", 0.5, 0.08, "Gray,FontAwesome" );
  btnConnect.SetMargins( 0.1, 0.01, 0.1, 0.01 );
  btnConnect.SetTextSize( 24 );
  btnConnect.SetOnTouch( btnConnect_OnTouch );  
  lay.AddChild( btnConnect );
 
  app.AddLayout( lay );
 
  nxt = app.CreateNxt();
  nxt.SetOnConnected(onNXTConnected);
}	
 
function onNXTConnected() 
{
    if (nxt.IsConnected()) {
        btnConnect.SetText("[fa-unlink] Disconnect");
        app.ShowPopup("Connected to Bluetooth device: "+nxt.GetBtName()+" : "+nxt.GetBtAddress());
    }
}
 
function btnConnect_OnTouch()
{        
	if (nxt.IsConnected()) {
	    app.ShowPopup( "Disconnectig from Bluetooth device: "+nxt.GetBtName() );
	    nxt.Disconnect();
	    btnConnect.SetText("[fa-plug] Connect");
	} else {
	    //Show list of NXT devices.
	    nxt.ShowDevices();
	}
}