02 — The done command

Goal

Add a done property to each todo object and a completeTodo function that returns a new array with the target item marked done. Update formatList so the output shows [x] or [ ] markers.

Why a done flag?

The beginner addTodo creates { text } objects — simple, but there's nowhere to store completion state. Adding done: false by default (on creation) keeps the shape consistent for every function that follows.

Update addTodo (one line)

// todo.js — change addTodo so new items carry a done flag
function addTodo(todos, text) {
  return [...todos, { text, done: false }];
}

Add completeTodo

// todo.js — add below addTodo
function completeTodo(todos, index) {
  return todos.map((t, i) => (i === index ? { ...t, done: true } : t));
}

completeTodo never mutates the array — it returns a brand-new one with the target item spread-merged ({ ...t, done: true }). All other items are returned unchanged.

Update formatList

// todo.js — replace the existing formatList
function formatList(todos) {
  if (todos.length === 0) return "No todos yet.";
  return todos
    .map((t, i) => `${i + 1}. [${t.done ? "x" : " "}] ${t.text}`)
    .join("\n");
}

Updated module.exports

module.exports = { readTodos, writeTodos, addTodo, completeTodo, formatList };

Checkpoint

Open a Node REPL (node) and paste:

const { completeTodo } = require('./todo');
const result = completeTodo([{ text: 'a', done: false }], 0);
console.log(result[0].done); // true

Expected output: true