03 Functions and modules

Goal: write reusable functions and split code across files using CommonJS modules. Introduce the todo module that the rest of the ladder builds on.

Functions

A function groups code you want to reuse.

function greet(name) {
  return `Hello, ${name}!`;
}

console.log(greet("Alice"));   // Hello, Alice!

CommonJS modules

Node uses require / module.exports to share code between files.

Exporting:

// math.js
function add(a, b) {
  return a + b;
}

module.exports = { add };

Importing:

// main.js
const { add } = require('./math');
console.log(add(2, 3));   // 5

The todo module

Create todo.js with two pure functions — no file system yet, just logic:

// todo.js
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 = { addTodo, formatList };

Checkpoint

Create scratch.js in the same directory:

// scratch.js
const { addTodo, formatList } = require('./todo');

const todos = addTodo([], 'buy milk');
console.log(formatList(todos));   // 1. buy milk

node scratch.js   # 1. buy milk

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