JavaScript ESモジュール — import、exportと実世界のパターン

// utils/currency.js

// Named exports — explicit names, multiple per file
export function formatPrice(amount, currency = 'USD') {
  return new Intl.NumberFormat('en-US', { style: 'currency', currency }).format(amount);
}

export function parseCents(cents) {
  return cents / 100;
}

export const DEFAULT_CURRENCY = 'USD';

// Importing named exports — names must match
import { formatPrice, parseCents } from './utils/currency.js';

// You can rename on import
import { formatPrice as fmt } from './utils/currency.js';

// Import the entire namespace as an object
import * as Currency from './utils/currency.js';
Currency.formatPrice(49.99); // works

// components/ProductCard.js

// Default export — one per file, no name required
export default function ProductCard({ product }) {
  return `<div class="card">${product.name}</div>`;
}

// Importing a default export — you choose the name
import ProductCard from './components/ProductCard.js';
import Card        from './components/ProductCard.js'; // same thing, different name

// utils/index.js — the barrel
export { formatPrice, parseCents, DEFAULT_CURRENCY } from './currency.js';
export { validateEmail, validatePhone }              from './validation.js';
export { debounce, throttle }                        from './timing.js';
export { fetchWithRetry, buildQueryString }          from './http.js';

// Consumer — clean import path, no need to know the file structure
import { formatPrice, debounce, fetchWithRetry } from './utils/index.js';
// or with bundlers that resolve index.js automatically:
import { formatPrice, debounce, fetchWithRetry } from './utils';

// Load a heavy chart library only when the user navigates to the analytics page
async function loadAnalyticsDashboard() {
  const { Chart }   = await import('chart.js');
  const { buildDashboardData } = await import('./analytics/data.js');

  const data   = await buildDashboardData();
  const canvas = document.getElementById('dashboard-chart');
  new Chart(canvas, { type: 'bar', data });
}

// Attach to a route change
router.on('/analytics', loadAnalyticsDashboard);

// Feature flags — only load a module if the feature is enabled
async function initFeature(featureName) {
  const flags = await getFeatureFlags();

  if (flags[featureName]) {
    const module = await import(`./features/${featureName}.js`);
    module.init();
  }
}

// Get the directory of the current module (Node.js)
import { fileURLToPath } from 'node:url';
import { dirname, join }  from 'node:path';

const __filename = fileURLToPath(import.meta.url);
const __dirname  = dirname(__filename);

// Now you can do path resolution the same way you would with CommonJS
const configPath = join(__dirname, '../config/app.json');

// In Vite projects, import.meta.env gives you environment variables
const apiBase = import.meta.env.VITE_API_BASE_URL;
const isProd  = import.meta.env.PROD; // boolean

// import.meta.glob — Vite-specific, import multiple files at once
const modules = import.meta.glob('./pages/*.js');
for (const path in modules) {
  const module = await modules[path]();
  console.log(path, module);
}

// CommonJS (CJS) — still common in Node.js
const express = require('express');
const { join } = require('path');
module.exports = { myFunction };
module.exports.default = MyClass;

// ES Modules (ESM) — the standard
import express from 'express';
import { join } from 'path';
export { myFunction };
export default MyClass;

// ✅ Tree-shakeable — bundler can see exactly what's exported
export function formatDate(date) { /* ... */ }
export function parseDate(str)   { /* ... */ }
export function addDays(date, n) { /* ... */ }

// If you only import formatDate, parseDate and addDays are excluded from the bundle
import { formatDate } from './utils/dates.js';

// ❌ Harder to tree-shake — the whole object is one export
const dateUtils = {
  formatDate: (date) => { /* ... */ },
  parseDate:  (str)  => { /* ... */ },
  addDays:    (date, n) => { /* ... */ }
};
export default dateUtils;

// Even if you only use dateUtils.formatDate, the whole object may be bundled

<!-- index.html -->
<!-- type="module" enables ES Module semantics -->
<!-- deferred by default — runs after HTML is parsed -->
<script type="module" src="./app.js"></script>

// app.js — can use import/export directly
import { initRouter }    from './router.js';
import { setupAuth }     from './auth.js';
import { loadDashboard } from './dashboard.js';

async function main() {
  await setupAuth();
  initRouter();
  await loadDashboard();
}

main();

// Before: one giant utils.js with everything
export function formatPrice(amount) { /* ... */ }
export function validateEmail(email) { /* ... */ }
export function debounce(fn, ms) { /* ... */ }
export function fetchWithRetry(url, opts) { /* ... */ }
export function parseCSV(text) { /* ... */ }
export function buildQueryString(params) { /* ... */ }

// After: split by concern
// utils/
//   currency.js     → formatPrice, parseCents
//   validation.js   → validateEmail, validatePhone
//   timing.js       → debounce, throttle
//   http.js         → fetchWithRetry, buildQueryString
//   csv.js          → parseCSV, formatCSV
//   index.js        → re-exports all of the above (optional barrel)