Drop import of extensionless

doc/api/esm.md

@@ -33,9 +33,9 @@ initial input, or when referenced by `import` statements within 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`, or extensionless files when run as main entry points on
+  the command line, 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`.
@@ -50,9 +50,9 @@ 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`, or extensionless files when run as main entry points on
+  the command line, 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`.
@@ -1042,7 +1042,8 @@ a URL should be interpreted. This can be one of the following:
 ```js
 /**
  * @param {string} url
- * @param {object} context (currently empty)
+ * @param {object} context
+ * @param {string} context.parentURL
  * @param {function} defaultGetFormat
  * @returns {object} response
  * @returns {string} response.format
@@ -1366,13 +1367,15 @@ updates.
 In the following algorithms, all subroutine errors are propagated as errors
 of these top-level routines unless stated otherwise.
 
+_isMain_ is **true** when resolving the Node.js application entry point.
+
 _defaultEnv_ is the conditional environment name priority array,
 `["node", "import"]`.
 
 <details>
 <summary>Resolver algorithm specification</summary>
 
-**ESM_RESOLVE**(_specifier_, _parentURL_)
+**ESM_RESOLVE**(_specifier_, _parentURL_, _isMain_)
 
 > 1. Let _resolvedURL_ be **undefined**.
 > 1. If _specifier_ is a valid URL, then
@@ -1393,7 +1396,7 @@ _defaultEnv_ is the conditional environment name priority array,
 > 1. If the file at _resolvedURL_ does not exist, then
 >    1. Throw a _Module Not Found_ error.
 > 1. Set _resolvedURL_ to the real path of _resolvedURL_.
-> 1. Let _format_ be the result of **ESM_FORMAT**(_resolvedURL_).
+> 1. Let _format_ be the result of **ESM_FORMAT**(_resolvedURL_, _isMain_).
 > 1. Load _resolvedURL_ as module format, _format_.
 
 **PACKAGE_RESOLVE**(_packageSpecifier_, _parentURL_)
@@ -1546,20 +1549,20 @@ _defaultEnv_ is the conditional environment name priority array,
 >       1. Return _resolved_.
 > 1. Throw a _Module Not Found_ error.
 
-**ESM_FORMAT**(_url_)
+**ESM_FORMAT**(_url_, _isMain_)
 
-> 1. Assert: _url_ corresponds to an existing file pathname.
+> 1. Assert: _url_ corresponds to an existing file.
 > 1. Let _pjson_ be the result of **READ_PACKAGE_SCOPE**(_url_).
 > 1. If _url_ ends in _".mjs"_, then
 >    1. Return _"module"_.
 > 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 _isMain_ is **true** or _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. If _isMain_ is **true**, then
 >       1. Return _"commonjs"_.
 >    1. Throw an _Unsupported File Extension_ error.
 

lib/internal/modules/esm/get_format.js

@@ -54,13 +54,11 @@ function defaultGetFormat(url, context, defaultGetFormat) {
     return { format };
   } else if (parsed.protocol === 'file:') {
     const ext = extname(parsed.pathname);
-    let format;
-    if (ext === '.js' || ext === '') {
+    let format = extensionFormatMap[ext];
+    const isMain = context.parentURL === undefined;
+    if (ext === '.js' || (!format && isMain))
       format = getPackageType(parsed.href) === TYPE_MODULE ?
         'module' : 'commonjs';
-    } else {
-      format = extensionFormatMap[ext];
-    }
     if (!format) {
       if (experimentalSpeciferResolution === 'node') {
         process.emitWarning(

lib/internal/modules/esm/loader.js

@@ -96,7 +96,7 @@ class Loader {
     }
 
     const getFormatResponse = await this._getFormat(
-      url, {}, defaultGetFormat);
+      url, { parentURL }, defaultGetFormat);
     if (typeof getFormatResponse !== 'object') {
       throw new ERR_INVALID_RETURN_VALUE(
         'object', 'loader getFormat', getFormatResponse);