JavaScript implementation of set synchronisation. Bound by memory but minimizes database read writes.

sync.js

var _ = require('underscore')
  , a = {
    1: { id: '1', name: 'a' },
    2: { id: '2', name: 'b' },
    3: { id: '3', name: 'c' }
  }
  , b = {
    2: { id: '2', name: 'b1' },
    3: { id: '3', name: 'c2' },
    4: { id: '4', name: 'd' }
  };

function getId(item) {
  return item.id;
}

function isDifferent(first, second) {
  return Object.keys(first).some(function(key) {
    return first[key] !== second[key];
  });
}

function create(newObject) {
  a[newObject.id] = newObject;
}

function update(id, newObject) {
  a[id] = newObject;
}

function del(id) {
  delete a[id];
}

function sync(a, b, getId, isDifferent, create, update, del) {

  // Create sets
  var aIds = _.pluck(a, 'id')
    , bIds = _.pluck(b, 'id')
    , c = _.difference(aIds, bIds)
    , d = _.intersection(aIds, bIds)
    , e = _.difference(bIds, d);

  // Delete omitted items
  c.forEach(del);

  // Update only those that have changed
  d.forEach(function(index) {
    if (isDifferent(a[index], b[index])) {
      //update.push(b[index]);
      update(index, b[index]);
    }
  });

  // Insert new items
  e.forEach(function(index) {
    create(b[index]);
  });
}

sync(a, b, getId, isDifferent, create, update, del);
console.log(a);