onceAll.es6.js

function onceAll(events, callback) {
  var eventState = Object.create(null);
  
  // argument `events` is expected to be an array of event names.
  events.forEach((eventName) => {
    // Initil state: { event1: false, event2: false }
    eventState[eventName] = false; 
    
    // for each event, register a handler that updates the state and checks if we are 'done' yet
    this.once(eventName, () =>
      
      //mark it true when called once
      eventState[eventName] = true; 
      
      //are all events called, yet?
      if (events.every((eventName) => eventState[eventName])) {
        callback(); //success!
      }
  
    });
  });
}

Backbone.model.prototype.onceAll = onceAll; //or some other way to get this on there.

onceAll.js

function onceAll(events, callback) {
  var backboneObject = this;
  var eventState = Object.create(null);
  
  // argument `events` is expected to be an array of event names.
  events.forEach(function(eventName) {
    // Initil state: { event1: false, event2: false }
    eventState[eventName] = false; 
    
    // for each event, register a handler that updates the state and checks if we are 'done' yet
    // This is an IIFE to make sure 'eventName' is correct for each iteration of the loop
    (backboneObject.once(eventName, function() {
      
      //mark it true when called once
      eventState[eventName] = true; 
      
      //are all events called, yet?
      if (events.every(function(eventName) { return eventState[eventName]; })) {
        callback(); //success!
      }
  
    }))(eventName);
  });
}

Backbone.model.prototype.onceAll = onceAll; //or some other way to get this on there.