04 Read and write JSON

Goal: persist todos to a file using Node's built-in node:fs module and extend todo.js with readTodos and writeTodos.

JSON basics

JSON.stringify converts a JavaScript value to a JSON string; JSON.parse converts it back.

const todos = [{ text: "buy milk", done: false }];
const json = JSON.stringify(todos, null, 2);  // pretty-print
console.log(json);
// [
//   {
//     "text": "buy milk",
//     "done": false
//   }
// ]

const parsed = JSON.parse(json);
console.log(parsed[0].text);   // buy milk

Reading and writing files

node:fs provides synchronous file operations. Using the node: prefix is preferred for built-in modules:

const fs = require("node:fs");

// write
fs.writeFileSync("data.json", JSON.stringify([]), "utf8");

// read
const raw = fs.readFileSync("data.json", "utf8");
console.log(JSON.parse(raw));   // []

Extend the todo module

Add readTodos and writeTodos to todo.js:

// todo.js
const fs = require("node:fs");

function readTodos(path) {
  try {
    return JSON.parse(fs.readFileSync(path, "utf8"));
  } catch {
    return []; // missing or invalid file → start empty
  }
}

function writeTodos(path, todos) {
  fs.writeFileSync(path, JSON.stringify(todos, null, 2));
}

function addTodo(todos, text) {
  return [...todos, { text, done: false }];
}

function formatList(todos) {
  if (todos.length === 0) return "No todos yet.";
  return todos.map((t, i) => `${i + 1}. ${t.text}`).join("\n");
}

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

Checkpoint

Create persist.js:

// persist.js
const { readTodos, writeTodos, addTodo } = require('./todo');

const FILE = 'todos.json';
writeTodos(FILE, addTodo(readTodos(FILE), 'buy milk'));
console.log(readTodos(FILE));
// [ { text: 'buy milk', done: false } ]

node persist.js
# [ { text: 'buy milk', done: false } ]

The todos round-trip through todos.json correctly.

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Next: 05 The add command