Files ending in .mjs, but show Error: Unexpected import statement in CJS module.

[bahuwang]

• Version: 14.13.0
• Platform: windows 10
• Subsystem:

What steps will reproduce the bug?

@angular/core module file. https://github.com/bahuwang/issue/blob/master/core.js
demo.mjs file. https://github.com/bahuwang/issue/blob/master/demo.mjs
Just one line of code:

import { EventEmitter } from './core.js';

It is an ES modules, not CommonJS.

internal/deps/cjs-module-lexer/lexer.js:882
      throw new Error('Unexpected import statement in CJS module.');
            ^

Error: Unexpected import statement in CJS module.
  at @:7:8
?[90m    at throwIfImportStatement (internal/deps/cjs-module-lexer/lexer.js:882:13)?[39m
?[90m    at parseSource (internal/deps/cjs-module-lexer/lexer.js:83:13)?[39m
?[90m    at parseCJS (internal/deps/cjs-module-lexer/lexer.js:38:5)?[39m
?[90m    at cjsPreparseModuleExports (internal/modules/esm/translators.js:193:34)?[39m
?[90m    at Loader.commonjsStrategy (internal/modules/esm/translators.js:143:35)?[39m
?[90m    at new ModuleJob (internal/modules/esm/module_job.js:38:41)?[39m
?[90m    at Loader.getModuleJob (internal/modules/esm/loader.js:245:11)?[39m
?[90m    at async ModuleWrap.<anonymous> (internal/modules/esm/module_job.js:53:21)?[39m
    at async Promise.all (index 0)
?[90m    at async link (internal/modules/esm/module_job.js:58:9)?[39m {
  loc: ?[33m109?[39m
}

[guybedford]

@bahuwang the problem here is that Node.js thinks that core.js is a CommonJS module not an ES module. To tell Node.js that it is an ES module you need to create a package.json with a "type": "module" field or you need to rename the file to use the mjs extension as core.mjs.

That the error message here didn't give the correction information is a bug though, I've posted a fix for a better message in #35426.