浏览器将如何处理 ES6 导入/导出语法

Question

I've been thinking around this question lot of days and i have decided to ask the experts.

How browsers will handle the new import/export syntax ? I mean: will the modules be loaded asynchronously ? Referencing only my main or entry file and browsers will lazy load the requiere modules.

Maybe am i missing or misunderstanding something about this new architecture ?

Thank you very much!

Regards.

Answer 1

This is standardized now and supported by all major modern browsers.

will the modules be loaded asynchronously?

Yes, with two options available; details below.

Referencing only my main or entry file and browsers will lazy load the requiere modules.

Not so much "lazy," but yes.

Enabling it

Details in the spec here and here (and possibly elsewhere).

To get this behavior, you specify that your script is a module by using type="module" :

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

or for inline scripts

<script type="module">
// ...module code here
</script>

That means that the script is parsed and handled per the Module definition in the JavaScript specification instead of per the Script definition, which means it can have imports (and exports).

Imports are resolved relative to the script 's URL (for modules loaded via a separate resource such as the main.js above, just like CSS) or relative to the document (for inline modules like the one above).

So for instance, if I have this in my document at http://example.com/index.html :

<script src="./handy/stuff/nifty.js" type="module"></script>

...and nifty.js contains

import Thingy from "./thingy.js";

...then the browser looks for http://example.com/handy/stuff/thingy.js , not http://example.com/thingy.js . Again, just like CSS imports.

Note that the ./ on that module specifier is required, just from "thingy.js" won't work. That's because bare specifiers are disallowed because they'll probably end up having a special meaning. (For instance, in Node.js, that's how you specify built-in modules, and modules installed in node_modules .) A module specifier must be a full URL, or a relative URL starting with / , ./ , or ../ .

Async

I said above that modules are loaded asynchronously, and there are two options available. This graphic from the spec says it best (see the spec for the latest copy of it):

As you can see, for type="module" scripts, if you don't put any special flag attributes on the script tag, all of the module's dependencies will be resolved and then the script will be run once parsing of the HTML is complete. If you include the async attribute, it may run sooner, before the HTML parsing is complete (for instance, if all the scripts are in cache). ( defer is not valid for modules.)

Answer 2

According to this post in Mozilla's website, it's up to the implementation:

Because the system doesn't specify how loading works, and because you can figure out all the dependencies ahead of time by looking at the import declarations in the source code, an implementation of ES6 is free to do all the work at compile time and bundle all your modules into a single file to ship them over the network!

This may change in the future, as it is still not fully standardized, but you can be sure that you will not need to add a script tag for every module. Some module loaders today bundle all the files for you, but that's may not be the case when the future will be live, as it will not have an advantage in performance in HTTP2.

You can read the ES6 specification of import here .