Find nearby beacons

Here you go.

It scans every 5 minutes for 2 seconds, then sets its own manufacturer data to Espruino's ID (0x590) with the found device's UUID - otherwise it advertises normally.

function advertiseBeacon(dev) {
  var uuid = new Uint8Array(dev.manufacturerData,2,16);
  NRF.setAdvertising({},{
    manufacturer:0x590, // espruino's manufacturer
    manufacturerData:uuid
  });
}

function doScan() {  
  NRF.findDevices(function(devs) {
    var found = undefined;
    devs.forEach(function(dev) {
      if (dev.manufacturer==76)
        found = dev;
    });
    
    if (found) advertiseBeacon(found); // advertise one we found
    else NRF.setAdvertising({},{}); // don't advertise any data
  }, 2000); // scan for 2 seconds  
};

setInterval(doScan, 5*60000); // 5 minutes