stores.js

const $names = createStore([])
const $completed = createStore([])

const $todos = combine(
  $names, 
  $completed, 
  (names, completed) => names.map((name, i) => ({ name, completed: completed[i] })
)
  
$names
  .on(addTodo, (state, { name }) => [...state, name])

$completed
  .on(addTodo, (state, { completed }) => [...state, completed])
  .on(toggleTodo, (state, idx) => state.map((cur, i) => i === idx ? { ...cur, completed: !cur.completed } : cur))