simple observer

gistfile1.js

function watch(o, prop, fn) {
  var interval = 100 
  
  var value = o[prop]
  function check() {
    if(o[prop] != value) {
      fn(o[prop], value)
      value = o[prop]
    }
    
    setTimeout(check, interval)
  }
  
  check()
  
  return function(o) {
    interval = o.interval
  }   
}