[v12.x backport] module: drop support for extensionless main entry points in esm

doc/api/esm.md

@@ -36,9 +36,8 @@ ES module code:
 
 * Files ending in `.mjs`.
 
-* Files ending in `.js`, or extensionless files, when the nearest parent
-  `package.json` file contains a top-level field `"type"` with a value of
-  `"module"`.
+* Files ending in `.js` when the nearest parent `package.json` file contains a
+  top-level field `"type"` with a value of `"module"`.
 
 * Strings passed in as an argument to `--eval` or `--print`, or piped to
   `node` via `STDIN`, with the flag `--input-type=module`.
@@ -53,18 +52,17 @@ or when referenced by `import` statements within ES module code:
 
 * Files ending in `.cjs`.
 
-* Files ending in `.js`, or extensionless files, when the nearest parent
-  `package.json` file contains a top-level field `"type"` with a value of
-  `"commonjs"`.
+* Files ending in `.js` when the nearest parent `package.json` file contains a
+  top-level field `"type"` with a value of `"commonjs"`.
 
 * Strings passed in as an argument to `--eval` or `--print`, or piped to
   `node` via `STDIN`, with the flag `--input-type=commonjs`.
 
 ### `package.json` `"type"` field
 
-Files ending with `.js` or lacking any extension will be loaded as ES modules
-when the nearest parent `package.json` file contains a top-level field `"type"`
-with a value of `"module"`.
+Files ending with `.js` will be loaded as ES modules when the nearest parent
+`package.json` file contains a top-level field `"type"` with a value of
+`"module"`.
 
 The nearest parent `package.json` is defined as the first `package.json` found
 when searching in the current folder, that folder’s parent, and so on up
@@ -84,14 +82,12 @@ node --experimental-modules my-app.js # Runs as ES module
 ```
 
 If the nearest parent `package.json` lacks a `"type"` field, or contains
-`"type": "commonjs"`, extensionless and `.js` files are treated as CommonJS.
-If the volume root is reached and no `package.json` is found,
-Node.js defers to the default, a `package.json` with no `"type"`
-field. "Extensionless" refers to file paths which do not contain
-an extension as opposed to optionally dropping a file extension in a specifier.
+`"type": "commonjs"`, `.js` files are treated as CommonJS. If the volume root is
+reached and no `package.json` is found, Node.js defers to the default, a
+`package.json` with no `"type"` field.
 
-`import` statements of `.js` and extensionless files are treated as ES modules
-if the nearest parent `package.json` contains `"type": "module"`.
+`import` statements of `.js` files are treated as ES modules if the nearest
+parent `package.json` contains `"type": "module"`.
 
 ```js
 // my-app.js, part of the same example as above
@@ -109,14 +105,13 @@ as ES modules and `.cjs` files are always treated as CommonJS.
 
 ### Package Scope and File Extensions
 
-A folder containing a `package.json` file, and all subfolders below that
-folder down until the next folder containing another `package.json`, is
-considered a _package scope_. The `"type"` field defines how `.js` and
-extensionless files should be treated within a particular `package.json` file’s
-package scope. Every package in a project’s `node_modules` folder contains its
-own `package.json` file, so each project’s dependencies have their own package
-scopes. A `package.json` lacking a `"type"` field is treated as if it contained
-`"type": "commonjs"`.
+A folder containing a `package.json` file, and all subfolders below that folder
+down until the next folder containing another `package.json`, is considered a
+_package scope_. The `"type"` field defines how `.js` files should be treated
+within a particular `package.json` file’s package scope. Every package in a
+project’s `node_modules` folder contains its own `package.json` file, so each
+project’s dependencies have their own package scopes. A `package.json` lacking a
+`"type"` field is treated as if it contained `"type": "commonjs"`.
 
 The package scope applies not only to initial entry points (`node
 --experimental-modules my-app.js`) but also to files referenced by `import`
@@ -1610,12 +1605,10 @@ _defaultEnv_ is the conditional environment name priority array,
 > 1. If _url_ ends in _".cjs"_, then
 >    1. Return _"commonjs"_.
 > 1. If _pjson?.type_ exists and is _"module"_, then
->    1. If _url_ ends in _".js"_ or lacks a file extension, then
+>    1. If _url_ ends in _".js"_, then
 >       1. Return _"module"_.
 >    1. Throw an _Unsupported File Extension_ error.
 > 1. Otherwise,
->    1. If _url_ lacks a file extension, then
->       1. Return _"commonjs"_.
 >    1. Throw an _Unsupported File Extension_ error.
 
 **READ_PACKAGE_SCOPE**(_url_)

lib/internal/modules/esm/get_format.js

@@ -55,7 +55,7 @@ function defaultGetFormat(url, context, defaultGetFormat) {
   } else if (parsed.protocol === 'file:') {
     const ext = extname(parsed.pathname);
     let format;
-    if (ext === '.js' || ext === '') {
+    if (ext === '.js') {
       format = getPackageType(parsed.href) === TYPE_MODULE ?
         'module' : 'commonjs';
     } else {

test/es-module/test-esm-unknown-or-no-extension.js

@@ -0,0 +1,36 @@
+'use strict';
+
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const { spawn } = require('child_process');
+const assert = require('assert');
+
+// In a "type": "module" package scope, files with unknown extensions or no
+// extensions should throw; both when used as a main entry point and also when
+// referenced via `import`.
+
+[
+  '/es-modules/package-type-module/noext-esm',
+  '/es-modules/package-type-module/imports-noext.mjs',
+  '/es-modules/package-type-module/extension.unknown',
+  '/es-modules/package-type-module/imports-unknownext.mjs',
+].forEach((fixturePath) => {
+  const entry = fixtures.path(fixturePath);
+  const child = spawn(process.execPath, ['--experimental-modules', entry]);
+  let stdout = '';
+  let stderr = '';
+  child.stderr.setEncoding('utf8');
+  child.stdout.setEncoding('utf8');
+  child.stdout.on('data', (data) => {
+    stdout += data;
+  });
+  child.stderr.on('data', (data) => {
+    stderr += data;
+  });
+  child.on('close', common.mustCall((code, signal) => {
+    assert.strictEqual(code, 1);
+    assert.strictEqual(signal, null);
+    assert.strictEqual(stdout, '');
+    assert.ok(stderr.indexOf('ERR_UNKNOWN_FILE_EXTENSION') !== -1);
+  }));
+});