pack.js

function pack(modules) {
  const modulesSource = modules.map(module => 
    `${module.id}: {
      factory: (module, require) => {
        ${module.source}
      },
      map: ${JSON.stringify(module.map)}
    }`
  ).join()

  return `(modules => {
    const require = id => {
      const { factory, map } = modules[id]
      const localRequire = name => require(map[name])
      const module = { exports: {} }

      factory(module, localRequire)

      return module.exports
    }

    require(0)
  })({ ${modulesSource} })`
}