Object.observe(obj, callback) 

Object.getNotifier(object).notify({ 
  object: obj,
  type: "synthetic"
}); 

• If objects start broadcasting new change types, will existing code break?
  

• Design the pattern for objects to describe complex changes more compactly
• Existing code should never be surprised with unknown change types
  
• Only observers who can understand the more compact description should receive it
• More 'simple-minded' observers can always understand changes in terms of (underlying) intrinsic property changes

var square = new Square(0, 0, 10, 10);

function simpleton() { // do simpleton stuff }
Object.observe(square, simpleton);

function smartypants() { // do smart stuff }
Object.observe(square, smartypants,  [‘new’, ‘updated’, ‘deleted’, 'embiggened']
); 

embiggin: function(int amount) {
  notifier.performChange('embiggened', () => {
    this.width *= amount;
    this.height *= amount;
  });

  notifier.notify({
    type: 'embiggened',
    amount: amount
  });
} 

square.embiggen(2); 

{
  object: square, type: 'updated',
  name: 'width', oldValue: 10
},
{
  object: square, type: 'updated',
  name: 'height', oldValue: 10
} 

{
  object: square, type: 'embiggened', amount: 2
} 

• Specified to use performChange('splice', ... )
  
• Push/Pop/Shift/Unshift/Splice
• Property index updates which extend length
• length updates which extend or truncate length
• reverse()? sort()?
• ‘splice’ changeRecord type:
  

Array.observe = function(obj, callback) {
  return Object.observe(obj, callback,
                        ['new', 'updated', 'deleted', 'splice']);} 

• Purpose is observing an Array's "vector-ness".
• If you care about an Array's "object-ness", use Object.observe.