module: support require()ing synchronous ESM graphs

This patch adds `require()` support for synchronous ESM graphs under
the flag `--experimental-require-module`

This is based on the the following design aspect of ESM:

- The resolution can be synchronous (up to the host)
- The evaluation of a synchronous graph (without top-level await) is
  also synchronous, and, by the time the module graph is instantiated
  (before evaluation starts), this is is already known.

If `--experimental-require-module` is enabled, and the ECMAScript
module being loaded by `require()` meets the following requirements:

- Explicitly marked as an ES module with a `"type": "module"` field in
  the closest package.json or a `.mjs` extension.
- Fully synchronous (contains no top-level `await`).

`require()` will load the requested module as an ES Module, and return
the module name space object.

```mjs
// point.mjs
export function distance(a, b) {
  return (b.x - a.x) ** 2 + (b.y - a.y) ** 2;
}
class Point {
  constructor(x, y) { this.x = x; this.y = y; }
}
export default Point;
`
```

```cjs
// [Module: null prototype] {
//   default: [class Point],
//   distance: [Function: distance]
// }
console.log(require('./point.mjs'));
```

If the module being `require()`'d contains top-level `await`, or the
module graph it `import`s contains top-level `await`,
`ERR_REQUIRE_ASYNC_MODULE` will be thrown.

If `--experimental-print-required-tla` is enabled, instead of throwing
`ERR_REQUIRE_ASYNC_MODULE` before evaluation, Node.js will evaluate the
module, try to locate the top-level awaits, and print their location to
help users fix them.

doc/api/cli.md

@@ -871,6 +871,18 @@ added: v11.8.0
 
 Use the specified file as a security policy.
 
+### `--experimental-require-module`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1.1 - Active Developement
+
+Supports loading a synchronous ES module graph in `require()`.
+
+See [Loading ECMAScript modules using `require()`][].
+
 ### `--experimental-sea-config`
 
 <!-- YAML
@@ -1578,6 +1590,18 @@ changes:
 
 Identical to `-e` but prints the result.
 
+### `--experimental-print-required-tla`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+This flag is only useful when `--experimental-require-module` is enabled.
+
+If the ES module being `require()`'d contains top-level await, this flag
+allows Node.js to evaluate the module, try to locate the
+top-level awaits, and print their location to help users fix them.
+
 ### `--prof`
 
 <!-- YAML
@@ -2512,6 +2536,8 @@ Node.js options that are allowed are:
 * `--experimental-network-imports`
 * `--experimental-permission`
 * `--experimental-policy`
+* `--experimental-print-required-tla`
+* `--experimental-require-module`
 * `--experimental-shadow-realm`
 * `--experimental-specifier-resolution`
 * `--experimental-top-level-await`
@@ -3016,6 +3042,7 @@ node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
 [ExperimentalWarning: `vm.measureMemory` is an experimental feature]: vm.md#vmmeasurememoryoptions
 [Fetch API]: https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API
 [File System Permissions]: permissions.md#file-system-permissions
+[Loading ECMAScript modules using `require()`]: modules.md#loading-ecmascript-modules-using-require
 [Module customization hooks]: module.md#customization-hooks
 [Module customization hooks: enabling]: module.md#enabling
 [Modules loaders]: packages.md#modules-loaders

doc/api/errors.md

@@ -2487,14 +2487,28 @@ Accessing `Object.prototype.__proto__` has been forbidden using
 [`Object.setPrototypeOf`][] should be used to get and set the prototype of an
 object.
 
-<a id="ERR_REQUIRE_ESM"></a>
+<a id="ERR_REQUIRE_ASYNC_MODULE"></a>
+
+### `ERR_REQUIRE_ASYNC_MODULE`
+
+> Stability: 1 - Experimental
+
+When trying to `require()` a [ES Module][] under `--experimental-require-module`,
+the module turns out to be asynchronous. That is, it contains top-level await.
+
+To see where the top-level await is, use
+`--experimental-print-required-tla` (this would execute the modules
+before looking for the top-level awaits). <a id="ERR_REQUIRE_ESM"></a>
 
 ### `ERR_REQUIRE_ESM`
 
 > Stability: 1 - Experimental
 
 An attempt was made to `require()` an [ES Module][].
 
+To enable `require()` for synchronous module graphs (without
+top-level `await`), use `--experimental-require-module`.
+
 <a id="ERR_SCRIPT_EXECUTION_INTERRUPTED"></a>
 
 ### `ERR_SCRIPT_EXECUTION_INTERRUPTED`

doc/api/modules.md

@@ -167,16 +167,50 @@ variable. Since the module lookups using `node_modules` folders are all
 relative, and based on the real path of the files making the calls to
 `require()`, the packages themselves can be anywhere.
 
-## The `.mjs` extension
+## Loading ECMAScript modules using `require()`
 
-Due to the synchronous nature of `require()`, it is not possible to use it to
-load ECMAScript module files. Attempting to do so will throw a
-[`ERR_REQUIRE_ESM`][] error. Use [`import()`][] instead.
-
-The `.mjs` extension is reserved for [ECMAScript Modules][] which cannot be
-loaded via `require()`. See [Determining module system][] section for more info
+Currently, if the flag `--experimental-require-module` is not used, loading
+an ECMAScript module using `require()` will throw a [`ERR_REQUIRE_ESM`][]
+error, and users need to use [`import()`][] instead. See
+[Determining module system][] section for more info
 regarding which files are parsed as ECMAScript modules.
 
+If `--experimental-require-module` is enabled, and the ECMAScript module being
+loaded by `require()` meets the following requirements:
+
+* Explicitly marked as an ES module with a `"type": "module"` field in
+  the closest package.json or a `.mjs` extension.
+* Fully synchronous (contains no top-level `await`).
+
+`require()` will load the requested module as an ES Module, and return
+the module name space object.
+
+```mjs
+// point.mjs
+export function distance(a, b) { return (b.x - a.x) ** 2 + (b.y - a.y) ** 2; }
+class Point {
+  constructor(x, y) { this.x = x; this.y = y; }
+}
+export default Point;
+```
+
+```cjs
+// [Module: null prototype] {
+//   default: [class Point],
+//   distance: [Function: distance]
+// }
+console.log(require('./point.mjs'));
+```
+
+If the module being `require()`'d contains top-level `await`, or the module
+graph it `import`s contains top-level `await`,
+[`ERR_REQUIRE_ASYNC_MODULE`][] will be thrown.
+
+If `--experimental-print-required-tla` is enabled, instead of throwing
+`ERR_REQUIRE_ASYNC_MODULE` before evaluation, Node.js will evaluate the
+module, try to locate the top-level awaits, and print their location to
+help users fix them.
+
 ## All together
 
 <!-- type=misc -->
@@ -187,6 +221,8 @@ the `require.resolve()` function.
 Putting together all of the above, here is the high-level algorithm
 in pseudocode of what `require()` does:
 
+<!-- TODO(joyeecheung): update this for --experimental-require-module-->
+
 <pre>
 require(X) from module at path Y
 1. If X is a core module,
@@ -1085,6 +1121,7 @@ This section was moved to
 [GLOBAL_FOLDERS]: #loading-from-the-global-folders
 [`"main"`]: packages.md#main
 [`"type"`]: packages.md#type
+[`ERR_REQUIRE_ASYNC_MODULE`]: errors.md#err_require_async_module
 [`ERR_REQUIRE_ESM`]: errors.md#err_require_esm
 [`ERR_UNSUPPORTED_DIR_IMPORT`]: errors.md#err_unsupported_dir_import
 [`MODULE_NOT_FOUND`]: errors.md#module_not_found

doc/api/packages.md

@@ -1371,7 +1371,7 @@ This field defines [subpath imports][] for the current package.
 [entry points]: #package-entry-points
 [folders as modules]: modules.md#folders-as-modules
 [import maps]: https://github.com/WICG/import-maps
-[load ECMASCript modules from CommonJS modules]: modules.md#the-mjs-extension
+[load ECMASCript modules from CommonJS modules]: modules.md#loading-ecmascript-modules-using-require
 [loader hooks]: esm.md#loaders
 [packages folder mapping]: https://github.com/WICG/import-maps#packages-via-trailing-slashes
 [self-reference]: #self-referencing-a-package-using-its-name

lib/internal/errors.js

@@ -210,6 +210,7 @@ class NodeAggregateError extends AggregateError {
 }
 
 const assert = require('internal/assert');
+const { getOptionValue } = require('internal/options');
 
 // Lazily loaded
 let util;
@@ -1686,12 +1687,16 @@ E('ERR_PERFORMANCE_MEASURE_INVALID_OPTIONS', '%s', TypeError);
 E('ERR_REQUIRE_ESM',
   function(filename, hasEsmSyntax, parentPath = null, packageJsonPath = null) {
     hideInternalStackFrames(this);
+    assert(!getOptionValue('--experimental-require-module'));
     let msg = `require() of ES Module ${filename}${parentPath ? ` from ${
       parentPath}` : ''} not supported.`;
+    const hint = '\nOr use --experimental-require-module if the module is synchronous ' +
+      '(contains no top-level await)';
     if (!packageJsonPath) {
       if (StringPrototypeEndsWith(filename, '.mjs'))
         msg += `\nInstead change the require of ${filename} to a dynamic ` +
             'import() which is available in all CommonJS modules.';
+      msg += hint;
       return msg;
     }
     const path = require('path');
@@ -1700,6 +1705,7 @@ E('ERR_REQUIRE_ESM',
     if (hasEsmSyntax) {
       msg += `\nInstead change the require of ${basename} in ${parentPath} to` +
         ' a dynamic import() which is available in all CommonJS modules.';
+      msg += hint;
       return msg;
     }
     msg += `\n${basename} is treated as an ES module file as it is a .js ` +
@@ -1710,6 +1716,7 @@ E('ERR_REQUIRE_ESM',
       'modules, or change "type": "module" to "type": "commonjs" in ' +
       `${packageJsonPath} to treat all .js files as CommonJS (using .mjs for ` +
       'all ES modules instead).\n';
+    msg += hint;
     return msg;
   }, Error);
 E('ERR_SCRIPT_EXECUTION_INTERRUPTED',