parcel-bundler/src/builtins/bundle-loader.js

var getBundleURL = require('./bundle-url').getBundleURL;

function loadBundlesLazy(bundles) {
  if (!Array.isArray(bundles)) {
    bundles = [bundles]
  }

  var id = bundles[bundles.length - 1];

  try {
    return Promise.resolve(require(id));
  } catch (err) {
    if (err.code === 'MODULE_NOT_FOUND') {
      return new LazyPromise(function (resolve, reject) {
        loadBundles(bundles.slice(0, -1))
          .then(function () {
            return require(id);
          })
          .then(resolve, reject);
      });
    }

    throw err;
  }
}

function loadBundles(bundles) {
  return Promise.all(bundles.map(loadBundle));
}

var bundleLoaders = {};
function registerBundleLoader(type, loader) {
  bundleLoaders[type] = loader;
}

module.exports = exports = loadBundlesLazy;
exports.load = loadBundles;
exports.register = registerBundleLoader;

var bundles = {};
function loadBundle(bundle) {
  var id;
  if (Array.isArray(bundle)) {
    id = bundle[1];
    bundle = bundle[0];
  }

  if (bundles[bundle]) {
    return bundles[bundle];
  }

  var type = (bundle.substring(bundle.lastIndexOf('.') + 1, bundle.length) || bundle).toLowerCase();
  var bundleLoader = bundleLoaders[type];
  if (bundleLoader) {
    return bundles[bundle] = bundleLoader(getBundleURL() + bundle)
      .then(function (resolved) {
        if (resolved) {
          module.bundle.register(id, resolved);
        }

        return resolved;
      }).catch(function(e) {
        delete bundles[bundle];
        
        throw e;
      });
  }
}

function LazyPromise(executor) {
  this.executor = executor;
  this.promise = null;
}

LazyPromise.prototype.then = function (onSuccess, onError) {
  if (this.promise === null) this.promise = new Promise(this.executor)
  return this.promise.then(onSuccess, onError)
};

LazyPromise.prototype.catch = function (onError) {
  if (this.promise === null) this.promise = new Promise(this.executor)
  return this.promise.catch(onError)
};