Why am I getting a warning about synthetic events in my React Redux reducer?

Question

I'm trying to do this in a reducer;

case actions.DATA_RETURNED:
  newState = Object.assign({}, state);
  newState.status = 'complete';
  if (resp) {
    var newItems = newState.items.map(item => {
      var newItem = Object.assign({}, item);
      var match = _.find(resp, { id: item._id });
      newItem._rev = match.rev;
      return newItem;
    });
   }
  break;

I've tried several variations and they all fail with;

warning.js:36 Warning: This synthetic event is reused for performance reasons. If you're seeing this, you're accessing the property cancelable on a released/nullified synthetic event. This is set to null. If you must keep the original synthetic event around, use event.persist(). See https://facebook.github.io/react/docs/events.html#event-pooling for more information.

unless I remove newItem._rev = match.rev or set match to be a hard-coded string. So it seems like the problem is with trying to use lodash's find inside of Array.map in a reducer, but I don't understand why that's a problem. I'm not consciously doing anything with events, so I'm having a hard time working around it to find a solution.

Edit: in case it's relevant, here's how I dispatch the action;

return syncChangedItemsToDB(itemsChanged).then((resp) => {
  dispatch({type: actions.DATA_RETURNED, resp});
});

Does anyone know what's causing this warning and how I can work around it?

Answer 1

If I use Object.assign to copy the results of lodash's find , things seem to work with no problem, so this code works. I'm still not sure why that's necessary, so any explanation would be appreciated.

newItems = newState.items.map(item => {
  var match = Object.assign({}, _.find(resp, { id: item._id }));
  if (match.rev) item._rev = match.rev;
  return item;
});