Function populateCJSExportsFromESM

lib/internal/modules/cjs/loader.js:1775–1824

* Populate the exports of a CJS module entry from an ESM module's namespace object for * require(esm). * @param {Module} mod CJS module instance * @param {ModuleWrap} wrap ESM ModuleWrap instance. * @param {object} namespace The ESM namespace object.

(mod, wrap, namespace)

 * @param {object} namespace The ESM namespace object.
 */
function populateCJSExportsFromESM(mod, wrap, namespace) {
  // Tooling in the ecosystem have been using the __esModule property to recognize
  // transpiled ESM in consuming code. For example, a 'log' package written in ESM:
  //
  // export default function log(val) { console.log(val); }
  //
  // Can be transpiled as:
  //
  // exports.__esModule = true;
  // exports.default = function log(val) { console.log(val); }
  //
  // The consuming code may be written like this in ESM:
  //
  // import log from 'log'
  //
  // Which gets transpiled to:
  //
  // const _mod = require('log');
  // const log = _mod.__esModule ? _mod.default : _mod;
  //
  // So to allow transpiled consuming code to recognize require()'d real ESM
  // as ESM and pick up the default exports, we add a __esModule property by
  // building a source text module facade for any module that has a default
  // export and add .__esModule = true to the exports. This maintains the
  // enumerability of the re-exported names and the live binding of the exports,
  // without incurring a non-trivial per-access overhead on the exports.
  //
  // The source of the facade is defined as a constant per-isolate property
  // required_module_default_facade_source_string, which looks like this
  //
  // export * from 'original';
  // export { default } from 'original';
  // export const __esModule = true;
  //
  // And the 'original' module request is always resolved by
  // createRequiredModuleFacade() to `wrap` which is a ModuleWrap wrapping
  // over the original module.

  // We don't do this to modules that are marked as CJS ESM or that
  // don't have default exports to avoid the unnecessary overhead.
  // If __esModule is already defined, we will also skip the extension
  // to allow users to override it.
  if (ObjectHasOwn(namespace, 'module.exports')) {
    mod.exports = namespace['module.exports'];
  } else if (!ObjectHasOwn(namespace, 'default') || ObjectHasOwn(namespace, '__esModule')) {
    mod.exports = namespace;
  } else {
    mod.exports = createRequiredModuleFacade(wrap);
  }
}

/**
 * Wraps the given content in a script and runs it in a new context.