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                const initialTodos = [
  {name: "todo 1", done: false},
  {name: "todo 2", done: false},

function done(todos, index) {  

  // ここを書く
  todos[0].done = true
  return todos

0個目の task を done = true にする場合を考える.

immutable に変更できている場合は "initialTodos 1回目" と "initialTodos 2回目" には同じ値が出力されるはず.

console.log(new Date().toLocaleTimeString(), "----------------------------------------")
// Object の値を書き換えると Console 上でも値が変わっちゃうので stringify してる
console.log("実行前 =>", JSON.stringify(initialTodos))
setTimeout(() => {
  const next = done(initialTodos, 0)
console.log("done todos =>", JSON.stringify(next[0]))
console.log("実行後 =>",  initialTodos)
console.log("Done できてる? =>", next[0].done === true ? "正解" : "間違い", "Immutable にできてる? =>", initialTodos[0].done === false ? "正解" : "間違い")
}, 4000)