--es-module-specifier-resolution=node not working from v13.0.1

[jaydenseric]

--es-module-specifier-resolution=node is documented for v13.x:

Here is the specific example that works in v13.0.0, but not in v13.0.1 or v13.1.0:

node --experimental-modules --es-module-specifier-resolution=node index

As you can see in the screenshot above, the error is:

internal/modules/cjs/loader.js:895
    throw err;
    ^

Error: Cannot find module '/Users/jaydenseric/Desktop/test/index'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:892:15)
    at Function.Module._load (internal/modules/cjs/loader.js:785:27)
    at Function.Module.runMain (internal/modules/cjs/loader.js:1143:12)
    at internal/main/run_main_module.js:16:11 {
  code: 'MODULE_NOT_FOUND',
  requireStack: []
}

[guybedford]

The fix for this would be to ensure that .mjs is included in require.extensions when this flag is enabled.

This is because the CJS loader is doing the main entry resolution, and does not include .mjs resolution by default in its require.extensions list.

A way to get it to work would be to write:

extensions.js

require._extensions['.mjs'] = require._extensions['.js'];

which would allow:

node -r extensions.js --es-module-specifier-resolution=node index

A PR to add this when the flag is set is welcome.

[pd4d10]

@guybedford Thanks for the explanation.

I tried to create a PR to fix this issue: #30574. Please correct me if I understand it wrong.

[dandv]

Has this flag been renamed to experimental-specifier-resolution?

[MylesBorins]

@dandv we have renamed the flag, but we continue to support --es-module-specifier-resolution=node which silently aliases to --experimnetal-specifier-resolution

[bolinfest]

@guybedford when I create extensions.js as you describe and run:

$ cat extensions.js
// https://github.com/nodejs/node/issues/30520
require._extensions['.mjs'] = require._extensions['.js'];
$ node -r extensions.js --es-module-specifier-resolution=node --experimental-modules lib/server/main.js

I get:

internal/modules/cjs/loader.js:979
  throw err;
  ^

Error: Cannot find module 'extensions.js'
Require stack:
- internal/preload
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:976:15)
    at Function.Module._load (internal/modules/cjs/loader.js:859:27)
    at Module.require (internal/modules/cjs/loader.js:1036:19)
    at Module._preloadModules (internal/modules/cjs/loader.js:1292:12)
    at loadPreloadModules (internal/bootstrap/pre_execution.js:448:5)
    at prepareMainThreadExecution (internal/bootstrap/pre_execution.js:71:3)
    at internal/main/run_main_module.js:7:1 {
  code: 'MODULE_NOT_FOUND',
  requireStack: [ 'internal/preload' ]
}

what am I missing?