JavaScript modules and a loader, systemjs

In this post, I will talk a little bit about how to write modular JavaScript codes and how to use the modules via a popular loader, systemjs. This post is accompanied by this demo project:

The very first piece of JavaScript code

Every JavaScript developer can understand the following code, if you are fine to call it “code”.

It’s nothing but a cool little trick to help a user select the entire text inside the input box. It was the very first piece of JavaScript code I wrote, I still remember, in 2000, almost 15 years ago. JavaScript can be simply used like that. In fact, 15 years ago, there were so many places on the web using this kind of scattered JavaScript to serve various purposes that can’t be achieved by only HTML (or table if you recall). The point is, JavaScript by that time was not something that you will treat as a first class web development technique. Java, PHP or ASP were, but not JavaScript.

Fast forward

15 years later, now days, JavaScript is not only the first citizen of the web front-end society, but also the cool kid for many other areas like server side (node.js) and even Internet of things. This piece of code onmouseover="" is never cool anymore, instead it is almost like a crime if you write it not just for kidding. JavaScript is not a scripting language anymore. In fact, all main stream JavaScript implementations are JIT compilation based or at least mix compilation and interpretation, for decent performance purpose.

We started to write thousands of lines of JavaScript codes in either one file or a set of files. These codes plus all the libraries (jQuery, Angular, just to name a few) are almost a sea of JavaScript codes, yet most of them are loaded into browser by using the plain old <script/> tags that we are all familiar with. But we all know the pain of using this tag and the consequences if not enough attention is paid when maintaining these tags, the ordering, in particular. And the round trips overhead the many <script/> tags will incur. You might argue that putting everything into a bundle or a few bundles solves the problem. But again, what about the churns you have to deal with the bundle definition files. You still have to be very careful with the ordering, etc.

The Popular module formats/loaders

So to resolve this JavaScript codes organization problem, the module concept had become more and more popular in the past several years, and eventually led to a few popular module formats and their loaders. One of them is CommonJS, the de facto module standard on node.js platform. The other is AMD which was invented for browser scenarios.

The CommonJS loading scenario is relatively straightforward because the modules/files are loaded directly from the local file system. It is naturally a sync operation. In fact, in node.js, it is just a require() function call, as shown below:

The AMD format, however, is a bit different. In a browser world, loading resources, e.g. scripts from an Internet server should always be asynchronous considering the IO latency. Also, the module codes need to be wrapped, usually in IIFE format, otherwise, they are all gonna be globals. Here is an example:

In order to load the scripts, a browser context aware loader is needed. RequireJS was the de facto AMD loader. It is a bit outdated now. I’ll show you why later. Today we have the new cool kid called systemjs. We’ll get back to systemjs’ details later in this post. Based on the above code snippets, we can see there are some clear pros and cons for each format.

CommonJS format is really nice in the sense that we don’t need to wrap things in a function call. And the node.js loader (require()) will take care of the exports holder as well. But the bad part is also obvious in that it doesn’t have async semantic in the loader. You can require anywhere in the code, but we really need it to be async for browser scenarios. AMD and its loaders support async very well, totally work, however, the syntax, the wrapper style is not ideal. It’s just half way to an ideal JavaScript module solution.

The ideal module solution, ES6/ES2015

The JavaScript community is moving very quickly lately. Particularly the ES6 or ES2015 had been approved with quite a few goodies in it. I personally think the new module format is the one with the biggest potential impacts to the web. With the new ES6 module format, we can re-write the codes above like this:

In the above code snippets, I’m also using the new ES6 class syntax which is another very nice feature. Back to the point of module format, you can see now we have a much cleaner way of defining modules and their dependencies. There is no need to wrap things in an IIFE anymore. There is also no need for the magic exports holder object either. With all these good aspects, what really left is that we need a loader to make this module format works. And the loader better work in a much less overhead way.

The systemjs loader

Systemjs is a loader that can support the new ES6 module format, perfectly. In addition to that, it supports not only the new format, but also all popular legacy formats, CommonJS, AMD, and even globals. Isn’t it awesome? In fact, it is even more powerful, I’ll show you why in below. For a detailed demo, please see this little github demo repository.

Firstly, systemjs can load various different module formats as I mentioned above. Though in reality, we don’t usually mix too many different formats in a project. What we really want is the ES6 module format support. It is amazing to have this supported even before the main stream browsers fully support it. It accomplishes this with the help of the transpilers, the popular ones are Babel, Traceur and our beloved TypeScript. What does this mean? This means that, you can write ES6 modules today and don’t need to worry about if the browsers support them or not, because the ES6 modules will be seamlessly transpiled down to ES5 which is fully supported today by all main stream browsers. The transpilation can happen on the fly in browser for development workflow. It can also and must happen during build/bundle time for production scenarios, of course.

Secondly, systemjs has plugin loaders supported, meaning that many other kinds of resources can be universally loaded via systemjs just like JavaScript. This pattern today is so popular and it is so easy to use to manage modular web client apps (aka SPA). For example, HTML template files can be loaded dynamically and also bundled together with the feature JavaScript components. So are the LESS/CSS files, they can be authored and bundled in the same modular way. And all load via systemjs seamlessly/happily.

Thirdly, systemjs is not alone. It is accompanied by two other awesome tools, JSPM and systemjs-builder. JSPM as the name suggests is a package manager for browser scenarios, similarly as npm to node.js. With JSPM’s support, you can easily consume both well designed npm packages, and  even the raw repositories on github.

Systemjs-builder is the build/bundle part of the systemjs story. Remember bundling is a way to overcome the HTTP 1.x header of line blocking issue that will eventually disappear once HTTP 2 is established by major Internet service providers. Bundling by that time will be an anti-patter that we would need to un-learn. Really the loader is the thing we are looking at and bundling is the nice necessary feature it carries to solve the reality issue. This is also the reason why I personally favor the loader concept/tool over the bundler solutions like webpack. Webpack is also an awesome tool, but I don’t see a clear future of it because it solves the problem in a not-very-correct way.


The momentum we are seeing in the JavaScript and web front-end community is very exciting. It is generating very good stuff right now. The ES6 module formats and systemjs are just two of the many. I wish this little post has been helpful for folks that are new to this new world. Again, please go check out this little github demo repository to get some practical experiences of how all these things work together, beautifully.

Till next time,


JavaScript modules and a loader, systemjs

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