Adding JavaScript modules to the web platform

One thing we’ve been meaning to do more of is tell our blog readers more about new features we’ve been working on across WHATWG standards. We have quite a backlog of exciting things that have happened, and I’ve been nominated to start off by telling you the story of <script type="module"> .

JavaScript modules have a long history. They were originally slated to be finalized in early 2015 (as part of the “ES2015” revision of the JavaScript specification), but as the deadline drew closer, it became clear that although the syntax was ready, the semantics of how modules load each other were still up in the air. This is a hard problem anyway, as it involves extensive integration between the JavaScript engine and its “host environment”—which could be either a web browser, or something else, like Node.js.

The compromise that was reached was to have the JavaScript specification specify the syntax of modules, but without any way to actually run them. The host environment, via a hook called HostResolveImportedModule, would be responsible for resolving module specifiers (the "x" in import x from "x" ) into module instances, by executing the modules and fetching their dependencies. And so a year went by with JavaScript modules not being truly implementable in web browsers, as while their syntax was specified, their semantics were not yet.

In the epic whatwg/html#433 pull request, we worked on specifying these missing semantics. This involved a lot of deep changes to the script execution pipeline, to better integrate with the modern JavaScript spec. The WHATWG community had to discuss subtle issues like how cross-origin module scripts were fetched, or how/whether the async , defer , and charset attributes applied. The end result can be seen in a number of places in the HTML Standard, most notably in the definition of the script element and the scripting processing model sections. At the request of the Edge team, we also added support for worker modules, which you can see in the section on creating workers. (This soon made it over to the service workers spec as well!) To wrap things up, we included some examples: a couple for <script type="module"> , and one for module workers.

Of course, specifying a feature is not the end; it also needs to be implemented! Right now there is active implementation work happening in all four major rendering engines, which (for the open source engines) you can follow in these bugs:

And there's more work to do on the spec side, too! There's ongoing discussion of how to add more advanced dynamic module-loading APIs, from something simple like a promise-returning self.importModule , all the way up to the experimental ideas being prototyped in the whatwg/loader repository.

We hope you find the addition of JavaScript modules to the HTML Standard as exciting as we do. And we'll be back to tell you more about other recent important changes to the world of WHATWG standards soon!