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 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. We don't do this to
modules that don't have default exports to avoid the unnecessary
overhead. This maintains the enumerability of the re-exported names
and the live binding of the exports.
The source of the facade is defined as a constant per-isolate property
required_module_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.
PR-URL: https://github.com/nodejs/node/pull/52166
Refs: https://github.com/nodejs/node/issues/52134
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
This patch:
1. Adds ESM syntax detection to compileFunctionForCJSLoader()
for --experimental-detect-module and allow it to emit the
warning for how to load ESM when it's used to parse ESM as
CJS but detection is not enabled.
2. Moves the ESM detection of --experimental-detect-module for
the entrypoint from executeUserEntryPoint() into
Module.prototype._compile() and handle it directly in the
CJS loader so that the errors thrown during compilation *and
execution* during the loading of the entrypoint does not
need to be bubbled all the way up. If the entrypoint doesn't
parse as CJS, and detection is enabled, the CJS loader will
re-load the entrypoint as ESM on the spot asynchronously using
runEntryPointWithESMLoader() and cascadedLoader.import(). This
is fine for the entrypoint because unlike require(ESM) we don't
the namespace of the entrypoint synchronously, and can just
ignore the returned value. In this case process.mainModule is
reset to undefined as they are not available for ESM entrypoints.
3. Supports --experimental-detect-module for require(esm).
PR-URL: https://github.com/nodejs/node/pull/52047
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
