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

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

function loadBundles(bundles) {
  var id = Array.isArray(bundles) ? bundles[bundles.length - 1] : bundles;

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

    throw err;
  }
}

module.exports = exports = loadBundles;

var bundles = {};
var bundleLoaders = {
  js: loadJSBundle,
  css: loadCSSBundle
};

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

  var type = bundle.match(/\.(.+)$/)[1].toLowerCase();
  var bundleLoader = bundleLoaders[type];
  if (bundleLoader) {
    return bundles[bundle] = bundleLoader(getBundleURL() + bundle);
  }
}

function loadJSBundle(bundle) {
  return new Promise(function (resolve, reject) {
    var script = document.createElement('script');
    script.async = true;
    script.type = 'text/javascript';
    script.charset = 'utf-8';
    script.src = bundle;
    script.onerror = function (e) {
      script.onerror = script.onload = null;
      reject(e);
    };

    script.onload = function () {
      script.onerror = script.onload = null;
      resolve();
    };

    document.getElementsByTagName('head')[0].appendChild(script);
  });
}

function loadCSSBundle(bundle) {
  return new Promise(function (resolve, reject) {
    var link = document.createElement('link');
    link.rel = 'stylesheet';
    link.href = bundle;
    link.onerror = function (e) {
      link.onerror = link.onload = null;
      reject(e);
    };

    link.onload = function () {
      link.onerror = link.onload = null;
      resolve();
    };

    document.getElementsByTagName('head')[0].appendChild(link);
  });
}

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

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

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