Node.js mocking example

mock-loader.mjs

const mocks = Object.create(null);
global.mock = function (_mocks) {
  Object.assign(mocks, _mocks);
};

export async function resolve (specifier, context, parentResolve) {
  if (mocks[specifier]) {
    return { url: 'mock:' + specifier };
  }
  return parentResolve(specifier, context);
}

export async function getFormat(url, context, parentGetFormat) {
  if (url.startsWith('mock:')) {
    return { format: 'dynamic' };
  }
  return parentGetFormat(url, context, parentGetFormat);
}

export async function dynamicInstantiate (url) {
  const obj = mocks[url.slice(5)];
  const exports = Object.keys(obj);
  return {
    exports,
    execute (ns) {
      for (const key of exports)
        ns[key].set(obj[key]);
    }
  }
}

run.sh

node --loader ./mock-loader.mjs test.mjs

# Alternatively, locate at node_modules/mock-loader/loader.mjs with a package.json "exports": "./loader.mjs" set
# Then use something like:
# NODE_OPTIONS="--loader mock-loader" node test.mjs

test.mjs

mock({
  express: {
    custom: 'express'
  }
});

import('express').then(x => console.log(x));

Can you clarify your point here further?

Jest mocks rely on transpiling instead of using any proper real-time esm mechanism. Essentially they are fundamentally incompatibile with Node.js esm implementation - they sidestep it completey as they are fully transpiling everything.