project function of map is called twice

[maiermic]

The bug cyclejs/todomvc-cycle#35 in the TODO-MVC example of Cycles.js is introduced by [email protected] and is still present in the current latest release [email protected]. As far as I can tell, the project function of map (returned by amendStateWithChildren(sources.DOM)) in TaskList/index.js#L66 is called twice for each event. You can check this assumption by adding a debug statement in line 66

let amendedState$ = state$.debug('state')
    .map(amendStateWithChildren(sources.DOM))
    .remember();

and a log statement between line 14 and 15:

function amendStateWithChildren(DOMSource) {
  return function (todosData) {
    console.log('amendStateWithChildren', todosData);
    return {
      ...

If you run the app you see that each time you add an item to the list the console logs

state: Object {list: Array[1], filter: ""}
amendStateWithChildren Object {list: Array[1], filter: ""}
amendStateWithChildren Object {list: Array[1], filter: ""}

or something similar. Like I said: For each event of state$, the project function passed to state$.map is called twice.

[staltz]

Interesting bug. It's definitely a bug, and definitely related to remember. But here's one interesting bit:

  return sourceTodosData$.map(sourceTodosData =>
    reducer$.fold((todosData, reducer) => reducer(todosData), sourceTodosData)
  ).flatten()
- .remember();

Removing that (in model()) fixes the issue, because then the debug('state') will also emit. The double emission is expected, it's app logic. The first emission is due to the fold seed, the second is due to changeRouteReducer$'s startWith.

Still, we need to fix remember() and MemoryStream.

[staltz]

Fixed in v7.0