import(cjs) and require(cjs) share a cache

Hiya! 👋

This might very well be on purpose, and if so feel free to close this issue.

For background, I'm currently working on ESM support in Jest. We implement our own resolution and module loading (including caching), so I'm writing some tests to try to make sure our semantics matches node's semantics.

I wrote the following test:

// stateful.cjs
'use strict';

let num = 0;

module.exports = function inc() {
  num++;
  return num;
};

// test.mjs
import * as assert from 'assert';
import {createRequire} from 'module';

async function main() {
  const require = createRequire(import.meta.url);

  const requiredStateful = require('./stateful.cjs');
  // delete require.cache[require.resolve('./stateful.cjs')];
  const {default: importedStateful} = await import('./stateful.cjs');

  assert.equal(importedStateful(), 1);
  assert.equal(importedStateful(), 2);
  assert.equal(requiredStateful(), 1);
  assert.equal(importedStateful(), 3);
  assert.equal(requiredStateful(), 2);
  assert.equal(requiredStateful(), 3);
}

main().catch(err => {
  process.exitCode = 1;

  console.error(err);
});

However, this test fails because the numbers are 1, 2, 3, 4, 5, 6, e.g. they share the same module rather than having their own.

I assumed that the module cache of require and import() would not be shared, based on

require.cache is not used by import. It has a separate cache.

From https://nodejs.org/api/esm.html#esm_no_require_cache

Note that if I swap the order from require then import to import then require (and uncomment the delete), the test behaves like I expect.

So doing import(cjs) populates require.cache which means require(cjs) loads the cached module, and require(cjs) populates the cache so import(cjs) reads it, but doing delete require.cache only affect require, not import.

If import populates and reads from require.cache, I'd expect it do respect deletions from that same property. What I would expect from reading the docs is that import would neither populate nor read from require.cache.

I ran these tests with no flags, using node v13.12.0.

Again, this might be expected behavior, and if so I'd be happy to provide a PR with a docs change specifying this. As a user I find it quite confusing, though

Start with the ESM documentation section on the separate require.cache cache and reproduce the issue's test.mjs and stateful.cjs examples on a current Node.js version. Confirm the observed relationship between import(), require(), and require.cache, then document the intended behavior and deletion semantics if the behavior is expected.