> But I'm confused about how it handles updates. Do you just rerender the whole thing?

Here is how I use it in such scenarios:

  elems = {}

  Weya elem: document.body, ->
   for k, v of data
    elems[k] = @div "#{v}"

  #when element k changes
  onChange = (k) ->
   elems[k].textContent = "#{data[k]}"

If the change is more complex Weya can again be used to render the content of the changed element (not the entire dom).