lazy-module-loader

Lazy Module Loader is a universal JavaScript module loader allowing to load modules with asynchronous resolution of their dependencies.

Intention

Lazy Module Loader had been widely used in Opera Desktop browser as a dedicated solution for modularity and bundling mechanism for Web UI code before the ES modules were fully supported.

Usage

Once the Lazy Module Loader script is loaded, it defines a global loader instance, which can be used as follows:

const module = await loader.require('my/module');

The requested module is loaded with all its required dependencies resolved.

Path resolution

By default the resource path is caluclated with simple rules:

No extension - the JS file extension is appended:

• my/module to my/module.js

Directory-like path - points to the main module in the directory:

• my/module/ to my/module/main.js

Custom extension - returns the unchanged path:

• my/module/base.css to my/module/base.css

Module format

Modules utilize the CommonJS format, they can define the async init() method which is used to inject references to dependencies.

let Sevice;
 
const Module = {
 
  async init() {
    Service = await loader.require('some/dependendency');
  }
 
  loadData() {
    return Service.loadSomeData();
  }
};
 
module.exports = Module;

Lazy loading

Modules can also define optional dependencies (in a form of symbols) to other modules which are loaded at runtime, if needed. This is particularly useful when no direct references are needed, for example:

const WelcomePage = loader.symbol('pages/welcome/');
const ContactPage = loader.symbol('pages/contact/');
 
class Router extends Component {
 
  render() {
    switch (this.props.page) {
      case 'contact':
        return ContactPage;
      default:
        return WelcomePage;
    }
  }
}
 
module.exports = Router;

The dependencies don't have to be loaded until the router component decides to render the particular page.

Preloading

In order to achieve the maximum performance and responsiveness at runtime, modules can be preloaded at startup, not to lazy-load them upon user actions.

 
const module = await loader.preload('my/module');

Such call will load recursively all the required and optional dependencies.

Bundling

Modules can also be bundled together with all their dependencies into a single file. What allows them to be loaded synchronously in a bunch.

require('./bundler.js');
 
bundler.generate('root/module', 'My Library', {
  prefix: 'some/prefix/',
});

Plugins

Loader can chain the interceptor plugins, to serve as a fallback to custom loaders.

const plugin = {
  path(key) {
    const path = super.path(key);
    if (someCondition) {
      return `http://www.example.com/module/${path}`;
    }
    return path;
  },
};
loader.use(plugin);

Registry

All the loaded modules are kept in a registry. It is global and can be accessed as follows:

loader.registry

The registry is a map allowing to access the module information and easily analyze the dependency graph. Each module can be checked for all its dependencies and clients referencing it.