refresh.coffee

load_product_catalog = (url, frequency, callback) -> 

  data = {}   # keep the data for the catalog
  json = ""   # keep the raw JSON

  # load the product catalog, possibly from cache
  # the browser will add the caching request headers for us
  load = ->
    request = new XMLHttpRequest()
    request.open "GET", url, true
    request.onload = update
    request.send()
  
  # response handler - we may be getting the responseText
  # from cache, which is why we keep the JSON around
  update = ->
    # don't parse the JSON unless it's changed
    if this.responseText != json
      json = this.responseText
      data = JSON.parse json
    callback data
    # reload the catalog every minute 
    setTimeout load, frequency

  # do the initial load so there's no delay the first time
  # someone tries to do a lookup
  load()