class PreFlightService {
	goToGate(flight) {
		if (flight instanceof Flight !== true) throw new Error('Must be a flight')

		if (flight.atGate) throw new Error('Flight is already at the gate');

		return Object.assign({}, flight, {
			atGate: true
        });
    }
	refuel() {

    }
	restockFood() {

    }
	onBoardFlightTeam() {

    }
	onBoardPassengers() {

    }
}

class Flight {
	constructor() {
		this.passengers = [];
		this.team = [];
		this.food = [];
		this.refueled = false;
		this.atGate = false;
    }
}

let service = new PreFlightService;
let flight = new Flight;

flight.atGate = true;

service.goToGate(flight)