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The module tag explained

ES6 modules make it possible to express module dependencies declaratively and fetch them asynchronously, without boilerplate asynchronous loading logic. Now, there must be a way in HTML to kick off the asynchronous loading of an application's modules. This could be done programmatically:

<script>
System.import("myapp").then(function(app) {
  // ...
});
</script>

But the module tag provides a way to eliminate the boilerplate at app top-level, too:

<module>
import app from "myapp";
...
</module>

In fact, the module tag provides an overall programming experience that is similar to script but improves it in a few key ways.

module is a better script

Here are a few ways in which the module tag makes for a better programming model:

  • Declarative imports. Top-level code can import from external modules.

  • Async. Unlike script, which makes you opt into async loading (and at least currently doesn't allow inline source with async or defer), the module tag always loads and executes asynchronously.

  • Everything everywhere is in a module. There's no need to write global code ever, so the programming model is uniform.

  • Top-level code is in a private scope. Unlike script, top-level variable bindings are local to the module defined in the current module tag. In particular this means no more defensive IIFEs at top level.

  • Strict by default. Unlike global code, modules are automatically in strict mode, which cleans up several warts in JavaScript (like with). It also fixes some interoperability nightmares of non-strict code, in particular the semantics of block-local function declarations.

The "mixed model" of app data

It's common for web apps to have uncached HTML source that loads uncacheable data by mixing it inline with source code. This is just as convenient with the module tag, but it's now also interoperable with (possibly highly cacheable) external modules. For example, an app might have most of its application logic in cacheable modules, but its startup logic embedded in HTML with uncacheable data baked in:

<!doctype html>
<html>
  <head>
    <module>
      import app from "myapp";
      app.preload({
        "site-data": [
          // lots of preloaded data ...
        ]
      });
    </module>
    <link rel="stylesheet" href="base.css">
  </head>
  <body>
    <!-- ... -->
  </body>
</html>

One reason this pattern is popular is that it's so convenient to code in templated frameworks like PHP or Rails:

app.preload(<%= escape_unicode(json) %>);

Of course, it's possible to do the same thing with blocking script tags or XHR and async logic, but the goal of module is to subsume the conveniences of script so programmers can comfortably move to an all-modules development model without loss of ergonomics, especially for popular idioms.

External module scripts

As a replacement for script, of course module also needs to allow loading module sources from external files:

<module src="main.js"></module>

Shouldn't JS modules live in separate source files?

In general, yes. But module is simply a better script, and script continues to have plenty of usage, such as the "mixed model" described above.

Is this intended to be used for packaging?

No. The TAG is currently working on a more general and high priority asset packaging solution (note that there's more work to do there -- I don't think the TAG documents are currently addressing all the important requirements). The driving purpose of module is top-level or bootstrapping code.

What about concatenation?

Concatenation is a packaging solution, but a limited one (see above).

That said, the ServiceWorker and ES6 Loader API's will allow custom code packaging formats and concatenation. But ultimately the better solution is enabling bundling of arbitrary assets (not just code) via URL's and extensible package formats.

If packages become a browser format, how can they be shared with the server?

ES6 modules separate the concept of module naming from deployment format through the Loader API. For example, you can write

import _ from "underscore";

in your module source, and configure each host environment differently to find "underscore" in the appropriate location, without changing the source code.

In fact, a packaging format can be reused in multiple host environments, since loaders can easily be configured (or even designed to work out of the box) with the browser packaging format.

What about XSS filters?

The only safe way to do XSS filtering is whitelisting. For whitelisting filters, the addition of a new HTML tag poses no risks, because the tag is disallowed by default.

Blacklisting filters are sadly prevalent -- sadly, because they're so easy to break. Spend any amount of time with a blacklist filter and you'll find ways to break it. In practice, blacklist filters that aren't being broken into aren't being broken into because no one has tried.

In fact, the web platform adds new sources of executable script all the time in the form of attributes (such as media events like onloadstart or onplaying). In fact, a common way to defeat many blacklist filters is to hide the "javascript:" pseudo URL scheme with HTML entity encodings, which doesn't even require finding new attributes.

In short, module is very unlikely to materially affect the relative security of any (already dangerously insecure) blacklist filters.

How do we get there from here?

The module tag will need to be implicitly CDATA (so code like x < 5 doesn't cause parsing issues), automatically styled not to display its source, and legal in the head of an HTML document. For all these reasons, the stopgap solution that can be used compatibly with legacy browsers is an equivalent script MIME type:

<script type="application/javascript+module">
...
</script>

This is fully backwards-compatible and enables polyfilling in legacy browsers. The semantics is identical to module. But in the longer run, once enough browsers support module, that will become suitable for production apps and will be the more ergonomic syntax.