returnedDrones.js

const returnedDrones = {}

function dispatchDrones(drones) {
  drones.forEach(d => {
    droneId = d.dispatch() // droneId contains a random but unique id
    returnedDrones[droneId] = false
  }
}

// run this function at end of day
function findMissing() {
  return (
    Object
      .keys(returnedDrones)
      .map(id => [id, returnedDrones[id]])
      .find(([id, isReturned]) => isReturned == false)
  )[0]
}