esm: phase two of new esm implementation

This PR updates the current `--experimental-modules` implementation
based on the work of the modules team  and reflects Phase 2 of our
new modules plan.

The largest differences from the current implementation include

* `packge.type` which can be either `module` or `commonjs`
  - `type: "commonjs"`:
    - `.js` is parsed as commonjs
    - default for entry point without an extension is commonjs
  - `type: "module"`:
    - `.js` is parsed as esm
    - does not support loading JSON or Native Module by default
    - default for entry point without an extension is esm
* `--entry-type=[mode]`
  - allows you set the type on entry point.
* A new file extension `.cjs`.
  - this is specifically to support importing commonjs in the
    `module` mode.
  - this is only in the esm loader, the commonjs loader remains
    untouched, but the extension will work in the old loader if you use
    the full file path.
* `--es-module-specifier-resolution=[type]`
  - options are `explicit` (default) and `node`
  - by default our loader will not allow for optional extensions in
    the import, the path for a module must include the extension if
    there is one
  - by default our loader will not allow for importing directories that
    have an index file
  - developers can use `--es-module-specifier-resolution=node` to
    enable the commonjs specifier resolution algorithm
  - This is not a “feature” but rather an implementation for
    experimentation. It is expected to change before the flag is
    removed
* `--experimental-json-loader`
  - the only way to import json when `"type": "module"`
  - when enable all `import 'thing.json'` will go through the
    experimental loader independent of mode
  - based on https://github.com/whatwg/html/issues/4315
* You can use `package.main` to set an entry point for a module
  - the file extensions used in main will be resolved based on the
    `type` of the module

Refs: https://github.com/nodejs/modules/blob/master/doc/plan-for-new-modules-implementation.md
Refs: https://github.com/GeoffreyBooth/node-import-file-specifier-resolution-proposal
Refs: https://github.com/nodejs/modules/pull/180
Refs: https://github.com/nodejs/ecmascript-modules/pull/6
Refs: https://github.com/nodejs/ecmascript-modules/pull/12
Refs: https://github.com/nodejs/ecmascript-modules/pull/28
Refs: https://github.com/nodejs/modules/issues/255
Refs: https://github.com/whatwg/html/issues/4315
Refs: https://github.com/w3c/webcomponents/issues/770
Co-authored-by: Myles Borins <MylesBorins@google.com>
Co-authored-by: John-David Dalton <john.david.dalton@gmail.com>
Co-authored-by: Evan Plaice <evanplaice@gmail.com>
Co-authored-by: Geoffrey Booth <webmaster@geoffreybooth.com>
Co-authored-by: Michaël Zasso <targos@protonmail.com>

PR-URL: https://github.com/nodejs/node/pull/26745
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>

4 files changed, 591 insertions, 61 deletions

diff --git a/doc/api/cli.md b/doc/api/cli.md
index 4c5c8fb618..ad7b65e90a 100644
--- a/doc/api/cli.md
+++ b/doc/api/cli.md
@@ -131,9 +131,43 @@ conjunction with native stack and other runtime environment data.
 added: v6.0.0
 -->
 
+### `--entry-type=type`
+<!-- YAML
+added: REPLACEME
+-->
+
+Used with `--experimental-modules`, this configures Node.js to interpret the
+initial entry point as CommonJS or as an ES module.
+
+Valid values are `"commonjs"` and `"module"`. The default is to infer from
+the file extension and the `"type"` field in the nearest parent `package.json`.
+
+Works for executing a file as well as `--eval`, `--print`, `STDIN`.
+
 Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with
 `./configure --openssl-fips`.)
 
+### `--es-module-specifier-resolution=mode`
+<!-- YAML
+added: REPLACEME
+-->
+
+To be used in conjunction with `--experimental-modules`. Sets the resolution
+algorithm for resolving specifiers. Valid options are `explicit` and `node`.
+
+The default is `explicit`, which requires providing the full path to a
+module. The `node` mode will enable support for optional file extensions and
+the ability to import a directory that has an index file.
+
+Please see [customizing esm specifier resolution][] for example usage.
+
+### `--experimental-json-modules`
+<!-- YAML
+added: REPLACEME
+-->
+
+Enable experimental JSON support for the ES Module loader.
+
 ### `--experimental-modules`
 <!-- YAML
 added: v8.5.0
@@ -927,9 +961,10 @@ greater than `4` (its current default value). For more information, see the
 [REPL]: repl.html
 [ScriptCoverage]: https://chromedevtools.github.io/devtools-protocol/tot/Profiler#type-ScriptCoverage
 [V8 JavaScript code coverage]: https://v8project.blogspot.com/2017/12/javascript-code-coverage.html
+[customizing esm specifier resolution]: esm.html#esm_customizing_esm_specifier_resolution_algorithm
 [debugger]: debugger.html
 [debugging security implications]: https://nodejs.org/en/docs/guides/debugging-getting-started/#security-implications
 [emit_warning]: process.html#process_process_emitwarning_warning_type_code_ctor
-[experimental ECMAScript Module]: esm.html#esm_loader_hooks
+[experimental ECMAScript Module]: esm.html#esm_resolve_hook
 [libuv threadpool documentation]: http://docs.libuv.org/en/latest/threadpool.html
 [remote code execution]: https://www.owasp.org/index.php/Code_Injection
diff --git a/doc/api/errors.md b/doc/api/errors.md
index 7e32ba1cfc..0161853774 100644
--- a/doc/api/errors.md
+++ b/doc/api/errors.md
@@ -844,6 +844,18 @@ provided.
 Encoding provided to `TextDecoder()` API was not one of the
 [WHATWG Supported Encodings][].
 
+<a id="ERR_ENTRY_TYPE_MISMATCH"></a>
+#### ERR_ENTRY_TYPE_MISMATCH
+
+> Stability: 1 - Experimental
+
+The `--entry-type=commonjs` flag was used to attempt to execute an `.mjs` file
+or a `.js` file where the nearest parent `package.json` contains
+`"type": "module"`; or
+the `--entry-type=module` flag was used to attempt to execute a `.cjs` file or
+a `.js` file where the nearest parent `package.json` either lacks a `"type"`
+field or contains `"type": "commonjs"`.
+
 <a id="ERR_FALSY_VALUE_REJECTION"></a>
 ### ERR_FALSY_VALUE_REJECTION
 
@@ -1267,6 +1279,11 @@ An invalid or unexpected value was passed in an options object.
 
 An invalid or unknown file encoding was passed.
 
+<a id="ERR_INVALID_PACKAGE_CONFIG"></a>
+### ERR_INVALID_PACKAGE_CONFIG
+
+An invalid `package.json` file was found which failed parsing.
+
 <a id="ERR_INVALID_PERFORMANCE_MARK"></a>
 ### ERR_INVALID_PERFORMANCE_MARK
 
@@ -1440,7 +1457,7 @@ strict compliance with the API specification (which in some cases may accept
 
 > Stability: 1 - Experimental
 
-An [ES6 module][] loader hook specified `format: 'dynamic'` but did not provide
+An [ES Module][] loader hook specified `format: 'dynamic'` but did not provide
 a `dynamicInstantiate` hook.
 
 <a id="ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST"></a>
@@ -1449,13 +1466,6 @@ a `dynamicInstantiate` hook.
 A `MessagePort` was found in the object passed to a `postMessage()` call,
 but not provided in the `transferList` for that call.
 
-<a id="ERR_MISSING_MODULE"></a>
-### ERR_MISSING_MODULE
-
-> Stability: 1 - Experimental
-
-An [ES6 module][] could not be resolved.
-
 <a id="ERR_MISSING_PLATFORM_FOR_WORKER"></a>
 ### ERR_MISSING_PLATFORM_FOR_WORKER
 
@@ -1463,12 +1473,12 @@ The V8 platform used by this instance of Node.js does not support creating
 Workers. This is caused by lack of embedder support for Workers. In particular,
 this error will not occur with standard builds of Node.js.
 
-<a id="ERR_MODULE_RESOLUTION_LEGACY"></a>
-### ERR_MODULE_RESOLUTION_LEGACY
+<a id="ERR_MODULE_NOT_FOUND"></a>
+### ERR_MODULE_NOT_FOUND
 
 > Stability: 1 - Experimental
 
-A failure occurred resolving imports in an [ES6 module][].
+An [ES Module][] could not be resolved.
 
 <a id="ERR_MULTIPLE_CALLBACK"></a>
 ### ERR_MULTIPLE_CALLBACK
@@ -1555,7 +1565,7 @@ A given value is out of the accepted range.
 
 > Stability: 1 - Experimental
 
-An attempt was made to `require()` an [ES6 module][].
+An attempt was made to `require()` an [ES Module][].
 
 <a id="ERR_SCRIPT_EXECUTION_INTERRUPTED"></a>
 ### ERR_SCRIPT_EXECUTION_INTERRUPTED
@@ -2220,10 +2230,17 @@ A non-specific HTTP/2 error has occurred.
 Used in the `repl` in case the old history file is used and an error occurred
 while trying to read and parse it.
 
+<a id="ERR_INVALID_REPL_TYPE"></a>
+#### ERR_INVALID_REPL_TYPE
+
+> Stability: 1 - Experimental
+
+The `--entry-type=...` flag is not compatible with the Node.js REPL.
+
 <a id="ERR_MISSING_DYNAMIC_INSTANTIATE_HOOK"></a>
 #### ERR_MISSING_DYNAMIC_INSTANTIATE_HOOK
 
-Used when an [ES6 module][] loader hook specifies `format: 'dynamic'` but does
+Used when an [ES Module][] loader hook specifies `format: 'dynamic'` but does
 not provide a `dynamicInstantiate` hook.
 
 <a id="ERR_STREAM_HAS_STRINGDECODER"></a>
@@ -2250,7 +2267,6 @@ size.
 This `Error` is thrown when a read is attempted on a TTY `WriteStream`,
 such as `process.stdout.on('data')`.
 
-
 [`'uncaughtException'`]: process.html#process_event_uncaughtexception
 [`--force-fips`]: cli.html#cli_force_fips
 [`Class: assert.AssertionError`]: assert.html#assert_class_assert_assertionerror
@@ -2293,7 +2309,7 @@ such as `process.stdout.on('data')`.
 [`subprocess.kill()`]: child_process.html#child_process_subprocess_kill_signal
 [`subprocess.send()`]: child_process.html#child_process_subprocess_send_message_sendhandle_options_callback
 [`zlib`]: zlib.html
-[ES6 module]: esm.html
+[ES Module]: esm.html
 [ICU]: intl.html#intl_internationalization_support
 [Node.js Error Codes]: #nodejs-error-codes
 [V8's stack trace API]: https://github.com/v8/v8/wiki/Stack-Trace-API
diff --git a/doc/api/esm.md b/doc/api/esm.md
index e81a69c6ed..ce616e1f42 100644
--- a/doc/api/esm.md
+++ b/doc/api/esm.md
@@ -5,39 +5,233 @@
 
 > Stability: 1 - Experimental
 
+## Introduction
+
 <!--name=esm-->
 
+ECMAScript modules are [the official standard format][] to package JavaScript
+code for reuse. Modules are defined using a variety of [`import`][] and
+[`export`][] statements.
+
+Node.js fully supports ECMAScript modules as they are currently specified and
+provides limited interoperability between them and the existing module format,
+[CommonJS][].
+
 Node.js contains support for ES Modules based upon the
-[Node.js EP for ES Modules][].
+[Node.js EP for ES Modules][] and the [ECMAScript-modules implementation][].
 
-Not all features of the EP are complete and will be landing as both VM support
-and implementation is ready. Error messages are still being polished.
+Expect major changes in the implementation including interoperability support,
+specifier resolution, and default behavior.
 
 ## Enabling
 
 <!-- type=misc -->
 
-The `--experimental-modules` flag can be used to enable features for loading
-ESM modules.
+The `--experimental-modules` flag can be used to enable support for
+ECMAScript modules (ES modules).
+
+## Running Node.js with an ECMAScript Module
 
-Once this has been set, files ending with `.mjs` will be able to be loaded
-as ES Modules.
+There are a few ways to start Node.js with an ES module as its input.
+
+### Initial entry point with an <code>.mjs</code> extension
+
+A file ending with `.mjs` passed to Node.js as an initial entry point will be
+loaded as an ES module.
 
 ```sh
 node --experimental-modules my-app.mjs
 ```
 
-## Features
+### <code>--entry-type=module</code> flag
 
-<!-- type=misc -->
+Files ending with `.js` or `.mjs`, or lacking any extension,
+will be loaded as ES modules when the `--entry-type=module` flag is set.
+
+```sh
+node --experimental-modules --entry-type=module my-app.js
+```
+
+For completeness there is also `--entry-type=commonjs`, for explicitly running
+a `.js` file as CommonJS. This is the default behavior if `--entry-type` is
+unspecified.
+
+The `--entry-type=module` flag can also be used to configure Node.js to treat
+as an ES module input sent in via `--eval` or `--print` (or `-e` or `-p`) or
+piped to Node.js via `STDIN`.
+
+```sh
+node --experimental-modules --entry-type=module --eval \
+  "import { sep } from 'path'; console.log(sep);"
+
+echo "import { sep } from 'path'; console.log(sep);" | \
+  node --experimental-modules --entry-type=module
+```
+
+### <code>package.json</code> <code>"type"</code> field
+
+Files ending with `.js` or `.mjs`, 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"`.
+
+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
+until the root of the volume is reached.
+
+<!-- eslint-skip -->
+```js
+// package.json
+{
+  "type": "module"
+}
+```
+
+```sh
+# In same folder as above package.json
+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.
+
+## 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"`.
+
+The package scope applies not only to initial entry points (`node
+--experimental-modules my-app.js`) but also to files referenced by `import`
+statements and `import()` expressions.
+
+```js
+// my-app.js, in an ES module package scope because there is a package.json
+// file in the same folder with "type": "module".
+
+import './startup/init.js';
+// Loaded as ES module since ./startup contains no package.json file,
+// and therefore inherits the ES module package scope from one level up.
+
+import 'commonjs-package';
+// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
+// lacks a "type" field or contains "type": "commonjs".
+
+import './node_modules/commonjs-package/index.js';
+// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
+// lacks a "type" field or contains "type": "commonjs".
+```
+
+Files ending with `.mjs` are always loaded as ES modules regardless of package
+scope.
+
+Files ending with `.cjs` are always loaded as CommonJS regardless of package
+scope.
+
+```js
+import './legacy-file.cjs';
+// Loaded as CommonJS since .cjs is always loaded as CommonJS.
+
+import 'commonjs-package/src/index.mjs';
+// Loaded as ES module since .mjs is always loaded as ES module.
+```
+
+The `.mjs` and `.cjs` extensions may be used to mix types within the same
+package scope:
+
+- Within a `"type": "module"` package scope, Node.js can be instructed to
+  interpret a particular file as CommonJS by naming it with a `.cjs` extension
+  (since both `.js` and `.mjs` files are treated as ES modules within a
+  `"module"` package scope).
+
+- Within a `"type": "commonjs"` package scope, Node.js can be instructed to
+  interpret a particular file as an ES module by naming it with an `.mjs`
+  extension (since both `.js` and `.cjs` files are treated as CommonJS within a
+  `"commonjs"` package scope).
+
+## Package Entry Points
+
+The `package.json` `"main"` field defines the entry point for a package,
+whether the package is included into CommonJS via `require` or into an ES
+module via `import`.
+
+<!-- eslint-skip -->
+```js
+// ./node_modules/es-module-package/package.json
+{
+  "type": "module",
+  "main": "./src/index.js"
+}
+```
+```js
+// ./my-app.mjs
+
+import { something } from 'es-module-package';
+// Loads from ./node_modules/es-module-package/src/index.js
+```
+
+An attempt to `require` the above `es-module-package` would attempt to load
+`./node_modules/es-module-package/src/index.js` as CommonJS, which would throw
+an error as Node.js would not be able to parse the `export` statement in
+CommonJS.
+
+As with `import` statements, for ES module usage the value of `"main"` must be
+a full path including extension: `"./index.mjs"`, not `"./index"`.
+
+If the `package.json` `"type"` field is omitted, a `.js` file in `"main"` will
+be interpreted as CommonJS.
+
+> Currently a package can define _either_ a CommonJS entry point **or** an ES
+> module entry point; there is no way to specify separate entry points for
+> CommonJS and ES module usage. This means that a package entry point can be
+> included via `require` or via `import` but not both.
+>
+> Such a limitation makes it difficult for packages to support both new versions
+> of Node.js that understand ES modules and older versions of Node.js that
+> understand only CommonJS. There is work ongoing to remove this limitation, and
+> it will very likely entail changes to the behavior of `"main"` as defined
+> here.
+
+## <code>import</code> Specifiers
+
+### Terminology
+
+The _specifier_ of an `import` statement is the string after the `from` keyword,
+e.g. `'path'` in `import { sep } from 'path'`. Specifiers are also used in
+`export from` statements, and as the argument to an `import()` expression.
+
+There are four types of specifiers:
+
+- _Bare specifiers_ like `'some-package'`. They refer to an entry point of a
+  package by the package name.
+
+- _Deep import specifiers_ like `'some-package/lib/shuffle.mjs'`. They refer to
+  a path within a package prefixed by the package name.
+
+- _Relative specifiers_ like `'./startup.js'` or `'../config.mjs'`. They refer
+  to a path relative to the location of the importing file.
+
+- _Absolute specifiers_ like `'file:///opt/nodejs/config.js'`. They refer
+  directly and explicitly to a full path.
+
+Bare specifiers, and the bare specifier portion of deep import specifiers, are
+strings; but everything else in a specifier is a URL.
 
-### Supported
+Only `file://` URLs are supported. A specifier like
+`'https://example.com/app.js'` may be supported by browsers but it is not
+supported in Node.js.
 
-Only the CLI argument for the main entry point to the program can be an entry
-point into an ESM graph. Dynamic import can also be used to create entry points
-into ESM graphs at runtime.
+Specifiers may not begin with `/` or `//`. These are reserved for potential
+future use. The root of the current volume may be referenced via `file:///`.
 
-#### import.meta
+## import.meta
 
 * {Object}
 
@@ -46,63 +240,128 @@ property:
 
 * `url` {string} The absolute `file:` URL of the module.
 
-### Unsupported
+## Differences Between ES Modules and CommonJS
 
-| Feature | Reason |
-| --- | --- |
-| `require('./foo.mjs')` | ES Modules have differing resolution and timing, use dynamic import |
+### Mandatory file extensions
+
+A file extension must be provided when using the `import` keyword. Directory
+indexes (e.g. `'./startup/index.js'`) must also be fully specified.
 
-## Notable differences between `import` and `require`
+This behavior matches how `import` behaves in browser environments, assuming a
+typically configured server.
 
-### No NODE_PATH
+### No <code>NODE_PATH</code>
 
 `NODE_PATH` is not part of resolving `import` specifiers. Please use symlinks
 if this behavior is desired.
 
-### No `require.extensions`
+### No <code>require</code>, <code>exports</code>, <code>module.exports</code>, <code>\_\_filename</code>, <code>\_\_dirname</code>
+
+These CommonJS variables are not available in ES modules.
+
+`require` can be imported into an ES module using
+[`module.createRequireFromPath()`][].
+
+An equivalent for `__filename` and `__dirname` is [`import.meta.url`][].
+
+### No <code>require.extensions</code>
 
 `require.extensions` is not used by `import`. The expectation is that loader
 hooks can provide this workflow in the future.
 
-### No `require.cache`
+### No <code>require.cache</code>
 
 `require.cache` is not used by `import`. It has a separate cache.
 
-### URL based paths
+### URL-based paths
 
-ESM are resolved and cached based upon [URL](https://url.spec.whatwg.org/)
-semantics. This means that files containing special characters such as `#` and
-`?` need to be escaped.
+ES modules are resolved and cached based upon
+[URL](https://url.spec.whatwg.org/) semantics. This means that files containing
+special characters such as `#` and `?` need to be escaped.
 
 Modules will be loaded multiple times if the `import` specifier used to resolve
 them have a different query or fragment.
 
 ```js
-import './foo?query=1'; // loads ./foo with query of "?query=1"
-import './foo?query=2'; // loads ./foo with query of "?query=2"
+import './foo.mjs?query=1'; // loads ./foo.mjs with query of "?query=1"
+import './foo.mjs?query=2'; // loads ./foo.mjs with query of "?query=2"
 ```
 
 For now, only modules using the `file:` protocol can be loaded.
 
-## Interop with existing modules
+## Interoperability with CommonJS
+
+### <code>require</code>
 
-All CommonJS, JSON, and C++ modules can be used with `import`.
+`require` always treats the files it references as CommonJS. This applies
+whether `require` is used the traditional way within a CommonJS environment, or
+in an ES module environment using [`module.createRequireFromPath()`][].
 
-Modules loaded this way will only be loaded once, even if their query
-or fragment string differs between `import` statements.
+To include an ES module into CommonJS, use [`import()`][].
 
-When loaded via `import` these modules will provide a single `default` export
-representing the value of `module.exports` at the time they finished evaluating.
+### <code>import</code> statements
+
+An `import` statement can reference either ES module or CommonJS JavaScript.
+Other file types such as JSON and Native modules are not supported. For those,
+use [`module.createRequireFromPath()`][].
+
+`import` statements are permitted only in ES modules. For similar functionality
+in CommonJS, see [`import()`][].
+
+The _specifier_ of an `import` statement (the string after the `from` keyword)
+can either be an URL-style relative path like `'./file.mjs'` or a package name
+like `'fs'`.
+
+Like in CommonJS, files within packages can be accessed by appending a path to
+the package name.
 
 ```js
-// foo.js
-module.exports = { one: 1 };
+import { sin, cos } from 'geometry/trigonometry-functions.mjs';
+```
+
+> Currently only the “default export” is supported for CommonJS files or
+> packages:
+>
+> <!-- eslint-disable no-duplicate-imports -->
+> ```js
+> import packageMain from 'commonjs-package'; // Works
+>
+> import { method } from 'commonjs-package'; // Errors
+> ```
+>
+> There are ongoing efforts to make the latter code possible.
 
-// bar.mjs
-import foo from './foo.js';
-foo.one === 1; // true
+### <code>import()</code> expressions
+
+Dynamic `import()` is supported in both CommonJS and ES modules. It can be used
+to include ES module files from CommonJS code.
+
+```js
+(async () => {
+  await import('./my-app.mjs');
+})();
 ```
 
+## CommonJS, JSON, and Native Modules
+
+CommonJS, JSON, and Native modules can be used with [`module.createRequireFromPath()`][].
+
+```js
+// cjs.js
+module.exports = 'cjs';
+
+// esm.mjs
+import { createRequireFromPath as createRequire } from 'module';
+import { fileURLToPath as fromURL } from 'url';
+
+const require = createRequire(fromURL(import.meta.url));
+
+const cjs = require('./cjs');
+cjs === 'cjs'; // true
+```
+
+## Builtin modules
+
 Builtin modules will provide named exports of their public API, as well as a
 default export which can be used for, among other things, modifying the named
 exports. Named exports of builtin modules are updated when the corresponding
@@ -132,7 +391,41 @@ fs.readFileSync = () => Buffer.from('Hello, ESM');
 fs.readFileSync === readFileSync;
 ```
 
-## Loader hooks
+## Experimental JSON Modules
+
+**Note: This API is still being designed and is subject to change.**
+
+Currently importing JSON modules are only supported in the `commonjs` mode
+and are loaded using the CJS loader. [WHATWG JSON modules][] are currently
+being standardized, and are experimentally supported by including the
+additional flag `--experimental-json-modules` when running Node.js.
+
+When the `--experimental-json-modules` flag is included both the
+`commonjs` and `module` mode will use the new experimental JSON
+loader. The imported JSON only exposes a `default`, there is no
+support for named exports. A cache entry is created in the CommonJS
+cache, to avoid duplication. The same object will be returned in
+CommonJS if the JSON module has already been imported from the
+same path.
+
+Assuming an `index.js` with
+
+<!-- eslint-skip -->
+```js
+import packageConfig from './package.json';
+```
+
+The `--experimental-json-modules` flag is needed for the module
+to work.
+
+```bash
+node --experimental-modules --entry-type=module index.js # fails
+node --experimental-modules --entry-type=module --experimental-json-modules index.js # works
+```
+
+## Experimental Loader hooks
+
+**Note: This API is currently being redesigned and will still change.**
 
 <!-- type=misc -->
 
@@ -173,11 +466,10 @@ module. This can be one of the following:
 
 | `format` | Description |
 | --- | --- |
-| `'esm'` | Load a standard JavaScript module |
-| `'cjs'` | Load a node-style CommonJS module |
-| `'builtin'` | Load a node builtin CommonJS module |
+| `'module'` | Load a standard JavaScript module |
+| `'commonjs'` | Load a Node.js CommonJS module |
+| `'builtin'` | Load a Node.js builtin module |
 | `'json'` | Load a JSON file |
-| `'addon'` | Load a [C++ Addon][addons] |
 | `'dynamic'` | Use a [dynamic instantiate hook][] |
 
 For example, a dummy loader to load JavaScript restricted to browser resolution
@@ -253,6 +545,184 @@ With the list of module exports provided upfront, the `execute` function will
 then be called at the exact point of module evaluation order for that module
 in the import tree.
 
+## Resolution Algorithm
+
+### Features
+
+The resolver has the following properties:
+
+* FileURL-based resolution as is used by ES modules
+* Support for builtin module loading
+* Relative and absolute URL resolution
+* No default extensions
+* No folder mains
+* Bare specifier package resolution lookup through node_modules
+
+### Resolver Algorithm
+
+The algorithm to load an ES module specifier is given through the
+**ESM_RESOLVE** method below. It returns the resolved URL for a
+module specifier relative to a parentURL, in addition to the unique module
+format for that resolved URL given by the **ESM_FORMAT** routine.
+
+The _"module"_ format is returned for an ECMAScript Module, while the
+_"commonjs"_ format is used to indicate loading through the legacy
+CommonJS loader. Additional formats such as _"wasm"_ or _"addon"_ can be
+extended in future updates.
+
+In the following algorithms, all subroutine errors are propagated as errors
+of these top-level routines.
+
+_isMain_ is **true** when resolving the Node.js application entry point.
+
+When using the `--entry-type` flag, it overrides the ESM_FORMAT result while
+providing errors in the case of explicit conflicts.
+
+<details>
+<summary>Resolver algorithm specification</summary>
+
+**ESM_RESOLVE(_specifier_, _parentURL_, _isMain_)**
+> 1. Let _resolvedURL_ be **undefined**.
+> 1. If _specifier_ is a valid URL, then
+>    1. Set _resolvedURL_ to the result of parsing and reserializing
+>       _specifier_ as a URL.
+> 1. Otherwise, if _specifier_ starts with _"/"_, then
+>    1. Throw an _Invalid Specifier_ error.
+> 1. Otherwise, if _specifier_ starts with _"./"_ or _"../"_, then
+>    1. Set _resolvedURL_ to the URL resolution of _specifier_ relative to
+>       _parentURL_.
+> 1. Otherwise,
+>    1. Note: _specifier_ is now a bare specifier.
+>    1. Set _resolvedURL_ the result of
+>       **PACKAGE_RESOLVE**(_specifier_, _parentURL_).
+> 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_, _isMain_).
+> 1. Load _resolvedURL_ as module format, _format_.
+
+PACKAGE_RESOLVE(_packageSpecifier_, _parentURL_)
+> 1. Let _packageName_ be *undefined*.
+> 1. Let _packageSubpath_ be *undefined*.
+> 1. If _packageSpecifier_ is an empty string, then
+>    1. Throw an _Invalid Specifier_ error.
+> 1. If _packageSpecifier_ does not start with _"@"_, then
+>    1. Set _packageName_ to the substring of _packageSpecifier_ until the
+>       first _"/"_ separator or the end of the string.
+> 1. Otherwise,
+>    1. If _packageSpecifier_ does not contain a _"/"_ separator, then
+>       1. Throw an _Invalid Specifier_ error.
+>    1. Set _packageName_ to the substring of _packageSpecifier_
+>       until the second _"/"_ separator or the end of the string.
+> 1. Let _packageSubpath_ be the substring of _packageSpecifier_ from the
+>    position at the length of _packageName_ plus one, if any.
+> 1. Assert: _packageName_ is a valid package name or scoped package name.
+> 1. Assert: _packageSubpath_ is either empty, or a path without a leading
+>    separator.
+> 1. If _packageSubpath_ contains any _"."_ or _".."_ segments or percent
+>    encoded strings for _"/"_ or _"\\"_ then,
+>    1. Throw an _Invalid Specifier_ error.
+> 1. If _packageSubpath_ is empty and _packageName_ is a Node.js builtin
+>    module, then
+>    1. Return the string _"node:"_ concatenated with _packageSpecifier_.
+> 1. While _parentURL_ is not the file system root,
+>    1. Let _packageURL_ be the URL resolution of "node_modules/"
+>       concatenated with _packageSpecifier_, relative to _parentURL_.
+>    1. Set _parentURL_ to the parent folder URL of _parentURL_.
+>    1. If the folder at _packageURL_ does not exist, then
+>       1. Set _parentURL_ to the parent URL path of _parentURL_.
+>       1. Continue the next loop iteration.
+>    1. Let _pjson_ be the result of **READ_PACKAGE_JSON**(_packageURL_).
+>    1. If _packageSubpath_ is empty, then
+>       1. Return the result of **PACKAGE_MAIN_RESOLVE**(_packageURL_,
+>          _pjson_).
+>    1. Otherwise,
+>       1. Return the URL resolution of _packageSubpath_ in _packageURL_.
+> 1. Throw a _Module Not Found_ error.
+
+PACKAGE_MAIN_RESOLVE(_packageURL_, _pjson_)
+> 1. If _pjson_ is **null**, then
+>    1. Throw a _Module Not Found_ error.
+> 1. If _pjson.main_ is a String, then
+>    1. Let _resolvedMain_ be the concatenation of _packageURL_, "/", and
+>       _pjson.main_.
+>    1. If the file at _resolvedMain_ exists, then
+>       1. Return _resolvedMain_.
+> 1. If _pjson.type_ is equal to _"module"_, then
+>    1. Throw a _Module Not Found_ error.
+> 1. Let _legacyMainURL_ be the result applying the legacy
+>    **LOAD_AS_DIRECTORY** CommonJS resolver to _packageURL_, throwing a
+>    _Module Not Found_ error for no resolution.
+> 1. If _legacyMainURL_ does not end in _".js"_ then,
+>    1. Throw an _Unsupported File Extension_ error.
+> 1. Return _legacyMainURL_.
+
+**ESM_FORMAT(_url_, _isMain_)**
+> 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 _isMain_ is **true** or _url_ ends in _".js"_, then
+>       1. Return _"module"_.
+>    1. Throw an _Unsupported File Extension_ error.
+> 1. Otherwise,
+>    1. If _isMain_ is **true** or _url_ ends in _".js"_, _".json"_ or
+>       _".node"_, then
+>       1. Return _"commonjs"_.
+>    1. Throw an _Unsupported File Extension_ error.
+
+READ_PACKAGE_SCOPE(_url_)
+> 1. Let _scopeURL_ be _url_.
+> 1. While _scopeURL_ is not the file system root,
+>    1. Let _pjson_ be the result of **READ_PACKAGE_JSON**(_scopeURL_).
+>    1. If _pjson_ is not **null**, then
+>       1. Return _pjson_.
+>    1. Set _scopeURL_ to the parent URL of _scopeURL_.
+> 1. Return **null**.
+
+READ_PACKAGE_JSON(_packageURL_)
+> 1. Let _pjsonURL_ be the resolution of _"package.json"_ within _packageURL_.
+> 1. If the file at _pjsonURL_ does not exist, then
+>    1. Return **null**.
+> 1. If the file at _packageURL_ does not parse as valid JSON, then
+>    1. Throw an _Invalid Package Configuration_ error.
+> 1. Return the parsed JSON source of the file at _pjsonURL_.
+
+</details>
+
+### Customizing ESM specifier resolution algorithm
+
+The current specifier resolution does not support all default behavior of
+the CommonJS loader. One of the behavior differences is automatic resolution
+of file extensions and the ability to import directories that have an index
+file.
+
+The `--es-module-specifier-resolution=[mode]` flag can be used to customize
+the extension resolution algorithm. The default mode is `explicit`, which
+requires the full path to a module be provided to the loader. To enable the
+automatic extension resolution and importing from directories that include an
+index file use the `node` mode.
+
+```bash
+$ node --experimental-modules index.mjs
+success!
+$ node --experimental-modules index #Failure!
+Error: Cannot find module
+$ node --experimental-modules --es-module-specifier-resolution=node index
+success!
+```
+
+[`export`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export
+[`import`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import
+[`import()`]: #esm_import-expressions
+[`import.meta.url`]: #esm_import_meta
+[`module.createRequireFromPath()`]: modules.html#modules_module_createrequirefrompath_filename
+[CommonJS]: modules.html
+[ECMAScript-modules implementation]: https://github.com/nodejs/modules/blob/master/doc/plan-for-new-modules-implementation.md
 [Node.js EP for ES Modules]: https://github.com/nodejs/node-eps/blob/master/002-es-modules.md
-[addons]: addons.html
+[WHATWG JSON modules]: https://github.com/whatwg/html/issues/4315
 [dynamic instantiate hook]: #esm_dynamic_instantiate_hook
+[the official standard format]: https://tc39.github.io/ecma262/#sec-modules
diff --git a/doc/node.1 b/doc/node.1
index 7bcb1edc59..e2ac998cab 100644
--- a/doc/node.1
+++ b/doc/node.1
@@ -119,6 +119,15 @@ Enable FIPS-compliant crypto at startup.
 Requires Node.js to be built with
 .Sy ./configure --openssl-fips .
 .
+.It Fl -entry-type Ns = Ns Ar type
+Set the top-level module resolution type.
+.
+.It Fl -es-module-specifier-resolution
+Select extension resolution algorithm for ES Modules; either 'explicit' (default) or 'node'
+.
+.It Fl -experimental-json-modules
+Enable experimental JSON interop support for the ES Module loader.
+.
 .It Fl -experimental-modules
 Enable experimental ES module support and caching modules.
 .