item.js

const createStore = () => {
  let items = [];
  const watchers = [];

  const add = item => {
    items.push(item);
    emit();
  };

  const complete = id => {
    items = items.map(item => {
      if (item.id === id) {
        item.complete = true;
      }
      return item;
    });
    emit();
  };

  const emit = () => watchers.forEach(cb => cb(items));

  const onChange = cb => {
    const i = watchers.length - 1;
    watchers.push(cb);
    cb(items)
    return () => watchers.splice(1, i);
  };

  return {
    items,
    add,
    complete,
    onChange
  };
};

const create = body => ({
  id: Math.floor(Math.random() * 100),
  body,
  complete: false
});