Compositional ESM Loader

Enabling

Enable the compositional loader using --loader and --experimental-import-meta-resolve

node --loader @bradleymeck/compositional-esm-loader --experimental-import-meta-resolve app.js

To compose loaders together use the LOADERS environment variable. This is currently a JSON Array of string specifiers. Specifiers will be imported relative to the current working directory.

LOADERS='[
  "./example/log-hooks.mjs",
  "@bradleymeck/compositional-esm-loader/example/time-hooks.mjs"
]' \
node \
  --loader @bradleymeck/compositional-esm-loader \
  --experimental-import-meta-resolve \
  ./example/main.mjs

Loaders will be called from left to right in a chained delegation form of composition. In the case above log-hooks.mjs will be called first as the "bottom loader" and time-hooks.mjs will be above it.

Terminology

The loader list can be thought of as a list going from a bottom loader to a top loader.

• bottom loader - the first loader called by node.
• top loader - the loader that performs the default behavior of node.
• above - A loader A is said to be "above" another loader B if it must be delegated to by the loader B which "below" it.
• below - A loader B is said to be "below" another loader A if it is able to call the behavior of loader A when performing a hook.

Creating Loaders

NOTE: dynamicInstantiate is DISABLED for composition.

All hooks can use their default function to call behavior to the loader above them.

// no-op.mjs
export function resolve(specifier, context, defaultResolve) {
  return defaultResolve(specifier, context);
}

If a loaders does not implement a hook, it will be skipped. A loader is thought of as implementing a hook if it has the appropriately named export, not if that export is a function.

// bad.mjs

// this will cause errors if this loader is called to resolve something
export let resolve = null;

Trust

Currently there is not a cohesive trust model amongst loader composition.

getSource and transformSource are patched to work with typed-arrays while a value to source text conversion bug is resolved. They work, but might have minor changes in this module and/or in node core.

This is to be worked on.

Loaders preempting others to gain privilege to APIs desiring to be censored needs more investigation.