mixing-promises.js

  class Thing {
    constructor(type) {
      this.type = type;
      this.conflicts = [];
    }

    conflict(that){
      if(this.type === 'water' && that.type === "fire"){
        this.conflicts.push({id: that.id, message: "fire and water don't mix"});
      }
    }
  }


  class ThingTracker {
    constructor(things) {
      this.things = things;
    }

    preflight(newThing){
      this.things.forEach(function(existingThing){
        newThing.conflict(existingThing);
      });

      if(magicalPromiseDetector){
        if(newThing.conflicts.length === 0){
          return Promise.resolve(newThing);
        } else {
          return Promise.reject(newThing.conflicts);
        }
      } else {
        return newThing.conflicts.length === 0;
      }
    }
  }

  //usage on server

  var newThing = new Thing(request.data.thing);
  var thingTracker = new ThingTracker(db.lookup(request.data.id));
  var valid = thingTracker.preflight(newThing);
  if(valid){
    db.save(newThing, request.data.id);
  } else {
    request.respond(400, newThing.conflicts)
  }

  //usage on client
  localTracker.preflight(newThing).then(function(newThing){
    return client.request.addThing(newThing);
  }).then(function(newThing){
    localTracker.save(newThing);
  }).catch(function(conflicts){
    ui.show(conflicts);
  });

  //mixing on client
  Promise.resolve().then(localTracker.preflight(newThing)).then(function(valid){
    if(valid){
      return client.request.addThing(newThing);
    } else {
      throw newThing.conflicts;
    }
  }).then(function(newThing){
    localTracker.save(newThing);
  }).catch(function(conflicts){
    ui.show(conflicts);
  });
  
  //sync on client
  new Promise(function(resolve, reject){
    var valid = localTracker.preflight(newThing);
    if(valid){
      return client.request.addThing(newThing);
    } else {
      reject(newThing.conflicts);
    }
  }).then(function(newThing){
    localTracker.save(newThing);
  }).catch(function(conflicts){
    ui.show(conflicts);
  });