Add content hash to imports Babel Plugin

This Babel plugin adds hashes to static and dynamic import module names for cache busting.

For example:

import foo from './js/foo.js';

import('./bar.js').then(bar => bar());

is transformed into

import foo from './js/foo.abcd1234.js?';

import('./bar.1234abcd.js').then(bar => bar());

This is intended for use in multipage web applications (more on that later), so it only works with relative module names (e.g., ./foo.js or ../bar/baz.js) with .js extension. Feel free to adjust to your needs, obviously.

The content hash is simply the first 8 characters of the file's md5.

Why?

Now that browsers support import natively, web developers can use it without transpilers or bundlers (like webpack).

This not only makes deploying code easier because less tooling is required. For multipage apps (MPAs), configuring dozens (or even hundreds) of entry points with splitChunks is a nightmare.

There is one major downside of using imports natively however, namely caching.

Mapping module name to URLs

Since module names for static imports must be, well, static, we can't change the URL that will be fetched.

In principe, we could improve dynamic imports with something like:

const VERSION = '1.2.3-abc';     // global constant
function cacheBustImport (mod) {
    return import(`${mod}?v=${VERSION}`);   // e.g. foo.js?v=1.2.3-abc
}

cacheBustImport('./foo.js')
    .then(mod => console.log(mod));

But this is nasty. Also, since this breaks static code analysis, IDEs and build tools won't be able to analyze the imports.

Caching

Since we can't add the build version or content hash to enable Cache-Control: immutable, we need to fall back to must-revalidate.

If we author code with deep dependency trees, this will cause a huge number of sequential requests. HTTP/2 helps performance significantly, especially when combined with modulepreload.

Unfortunately, browser support for modulepreload is (currently) awful, plus it does still require quite a bit of effort to make it work.

Solution: content hash in URL

The ideal solution is to add a content hash for each imported file. This is a very old trick, so I was very surprised when I couldn't find any solution for imports (other than bundlers).

Luckily, writing plugins for Babel is a breeze (I myself have no experience with compilers and wrote this in a few hours!).