module: Allow runMain to be ESM

This follows the EPS an allows the node CLI to have ESM as an entry point.
`node ./example.mjs`. A newer V8 is needed for `import()` so that is not
included. `import.meta` is still in specification stage so that also is not
included.

PR-URL: #14369
Author: Bradley Farias <bradley.meck@gmail.com>
Author: Guy Bedford <guybedford@gmail.com>
Author: Jan Krems <jan.krems@groupon.com>
Author: Timothy Gu <timothygu99@gmail.com>
Author: Michaël Zasso <targos@protonmail.com>
Author: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

.eslintrc.yaml

@@ -10,6 +10,11 @@ env:
 parserOptions:
   ecmaVersion: 2017
 
+overrides:
+  - files: ["doc/api/esm.md", "*.mjs"]
+    parserOptions:
+      sourceType: module
+
 rules:
   # Possible Errors
   # http://eslint.org/docs/rules/#possible-errors

Makefile

@@ -151,7 +151,7 @@ coverage-build: all
 		"$(CURDIR)/testing/coverage/gcovr-patches.diff"); fi
 	if [ -d lib_ ]; then $(RM) -r lib; mv lib_ lib; fi
 	mv lib lib_
-	$(NODE) ./node_modules/.bin/nyc instrument lib_/ lib/
+	$(NODE) ./node_modules/.bin/nyc instrument --extension .js --extension .mjs lib_/ lib/
 	$(MAKE)
 
 coverage-test: coverage-build
@@ -887,7 +887,7 @@ JSLINT_TARGETS = benchmark doc lib test tools
 
 jslint:
 	@echo "Running JS linter..."
-	$(NODE) tools/eslint/bin/eslint.js --cache --rulesdir=tools/eslint-rules --ext=.js,.md \
+	$(NODE) tools/eslint/bin/eslint.js --cache --rulesdir=tools/eslint-rules --ext=.js,.mjs,.md \
 	  $(JSLINT_TARGETS)
 
 jslint-ci:

doc/api/esm.md

@@ -0,0 +1,88 @@
+# ECMAScript Modules
+
+<!--introduced_in=v9.x.x-->
+
+> Stability: 1 - Experimental
+
+<!--name=esm-->
+
+Node contains support for ES Modules based upon the [the Node EP for ES Modules][].
+
+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.
+
+## Enabling
+
+<!-- type=misc -->
+
+The `--experimental-modules` flag can be used to enable features for loading ESM modules.
+
+Once this has been set, files ending with `.mjs` will be able to be loaded as ES Modules.
+
+```sh
+node --experimental-modules my-app.mjs
+```
+
+## Features
+
+<!-- type=misc -->
+
+### Supported
+
+Only the CLI argument for the main entry point to the program can be an entry point into an ESM graph. In the future `import()` can be used to create entry points into ESM graphs at run time.
+
+### Unsupported
+
+| Feature | Reason |
+| --- | --- |
+| `require('./foo.mjs')` | ES Modules have differing resolution and timing, use language standard `import()` |
+| `import()` | pending newer V8 release used in Node.js |
+| `import.meta` | pending V8 implementation |
+| Loader Hooks | pending Node.js EP creation/consensus |
+
+## Notable differences between `import` and `require`
+
+### No NODE_PATH
+
+`NODE_PATH` is not part of resolving `import` specifiers. Please use symlinks if this behavior is desired.
+
+### No `require.extensions`
+
+`require.extensions` is not used by `import`. The expectation is that loader hooks can provide this workflow in the future.
+
+### No `require.cache`
+
+`require.cache` is not used by `import`. It has a separate cache.
+
+### URL based paths
+
+ESM are resolved and cached based upon [URL](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"
+```
+
+For now, only modules using the `file:` protocol can be loaded.
+
+## Interop with existing modules
+
+All CommonJS, JSON, and C++ modules can be used with `import`.
+
+Modules loaded this way will only be loaded once, even if their query or fragment string differs between `import` statements.
+
+When loaded via `import` these modules will provide a single `default` export representing the value of `module.exports` at the time they finished evaluating.
+
+```js
+import fs from 'fs';
+fs.readFile('./foo.txt', (err, body) => {
+  if (err) {
+    console.error(err);
+  } else {
+    console.log(body);
+  }
+});
+```
+
+[the Node EP for ES Modules]: https://github.com/nodejs/node-eps/blob/master/002-es-modules.md

lib/internal/bootstrap_node.js

@@ -109,6 +109,13 @@
         'DeprecationWarning', 'DEP0062', startup, true);
     }
 
+    if (process.binding('config').experimentalModules) {
+      process.emitWarning(
+        'The ESM module loader is experimental.',
+        'ExperimentalWarning', undefined);
+    }
+
+
     // There are various modes that Node can run in. The most common two
     // are running from a script and running the REPL - but there are a few
     // others like the debugger or running --eval arguments. Here we decide

lib/internal/errors.js

@@ -211,11 +211,15 @@ E('ERR_IPC_DISCONNECTED', 'IPC channel is already disconnected');
 E('ERR_IPC_ONE_PIPE', 'Child process can have only one IPC pipe');
 E('ERR_IPC_SYNC_FORK', 'IPC cannot be used with synchronous forks');
 E('ERR_MISSING_ARGS', missingArgs);
+E('ERR_MISSING_MODULE', 'Cannot find module %s');
+E('ERR_MODULE_RESOLUTION_LEGACY', '%s not found by import in %s.' +
+  'Legacy behavior in require would have found it at %s');
 E('ERR_NAPI_CONS_FUNCTION', 'Constructor must be a function');
 E('ERR_NAPI_CONS_PROTOTYPE_OBJECT', 'Constructor.prototype must be an object');
 E('ERR_NO_CRYPTO', 'Node.js is not compiled with OpenSSL crypto support');
 E('ERR_NO_ICU', '%s is not supported on Node.js compiled without ICU');
 E('ERR_PARSE_HISTORY_DATA', 'Could not parse history data in %s');
+E('ERR_REQUIRE_ESM', 'Must use import to load ES Module: %s');
 E('ERR_SOCKET_ALREADY_BOUND', 'Socket is already bound');
 E('ERR_SOCKET_BAD_TYPE',
   'Bad socket type specified. Valid types are: udp4, udp6');

lib/internal/loader/Loader.js

@@ -0,0 +1,75 @@
+'use strict';
+
+const { URL } = require('url');
+const { getURLFromFilePath } = require('internal/url');
+
+const {
+  getNamespaceOfModuleWrap
+} = require('internal/loader/ModuleWrap');
+
+const ModuleMap = require('internal/loader/ModuleMap');
+const ModuleJob = require('internal/loader/ModuleJob');
+const resolveRequestUrl = require('internal/loader/resolveRequestUrl');
+const errors = require('internal/errors');
+
+function getBase() {
+  try {
+    return getURLFromFilePath(`${process.cwd()}/`);
+  } catch (e) {
+    e.stack;
+    // If the current working directory no longer exists.
+    if (e.code === 'ENOENT') {
+      return undefined;
+    }
+    throw e;
+  }
+}
+
+class Loader {
+  constructor(base = getBase()) {
+    this.moduleMap = new ModuleMap();
+    if (typeof base !== 'undefined' && base instanceof URL !== true) {
+      throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'base', 'URL');
+    }
+    this.base = base;
+  }
+
+  async resolve(specifier) {
+    const request = resolveRequestUrl(this.base, specifier);
+    if (request.url.protocol !== 'file:') {
+      throw new errors.Error('ERR_INVALID_PROTOCOL',
+                             request.url.protocol, 'file:');
+    }
+    return request.url;
+  }
+
+  async getModuleJob(dependentJob, specifier) {
+    if (!this.moduleMap.has(dependentJob.url)) {
+      throw new errors.Error('ERR_MISSING_MODULE', dependentJob.url);
+    }
+    const request = await resolveRequestUrl(dependentJob.url, specifier);
+    const url = `${request.url}`;
+    if (this.moduleMap.has(url)) {
+      return this.moduleMap.get(url);
+    }
+    const dependencyJob = new ModuleJob(this, request);
+    this.moduleMap.set(url, dependencyJob);
+    return dependencyJob;
+  }
+
+  async import(specifier) {
+    const request = await resolveRequestUrl(this.base, specifier);
+    const url = `${request.url}`;
+    let job;
+    if (this.moduleMap.has(url)) {
+      job = this.moduleMap.get(url);
+    } else {
+      job = new ModuleJob(this, request);
+      this.moduleMap.set(url, job);
+    }
+    const module = await job.run();
+    return getNamespaceOfModuleWrap(module);
+  }
+}
+Object.setPrototypeOf(Loader.prototype, null);
+module.exports = Loader;

lib/internal/loader/ModuleJob.js

@@ -0,0 +1,116 @@
+'use strict';
+
+const { SafeSet, SafePromise } = require('internal/safe_globals');
+const resolvedPromise = SafePromise.resolve();
+const resolvedArrayPromise = SafePromise.resolve([]);
+const { ModuleWrap } = require('internal/loader/ModuleWrap');
+
+const NOOP = () => { /* No-op */ };
+class ModuleJob {
+  /**
+   * @param {module: ModuleWrap?, compiled: Promise} moduleProvider
+   */
+  constructor(loader, moduleProvider, url) {
+    this.url = `${moduleProvider.url}`;
+    this.moduleProvider = moduleProvider;
+    this.loader = loader;
+    this.error = null;
+    this.hadError = false;
+
+    if (moduleProvider instanceof ModuleWrap !== true) {
+      // linked == promise for dependency jobs, with module populated,
+      // module wrapper linked
+      this.modulePromise = this.moduleProvider.createModule();
+      this.module = undefined;
+      const linked = async () => {
+        const dependencyJobs = [];
+        this.module = await this.modulePromise;
+        this.module.link(async (dependencySpecifier) => {
+          const dependencyJobPromise =
+            this.loader.getModuleJob(this, dependencySpecifier);
+          dependencyJobs.push(dependencyJobPromise);
+          const dependencyJob = await dependencyJobPromise;
+          return dependencyJob.modulePromise;
+        });
+        return SafePromise.all(dependencyJobs);
+      };
+      this.linked = linked();
+
+      // instantiated == deep dependency jobs wrappers instantiated,
+      //module wrapper instantiated
+      this.instantiated = undefined;
+    } else {
+      const getModuleProvider = async () => moduleProvider;
+      this.modulePromise = getModuleProvider();
+      this.moduleProvider = { finish: NOOP };
+      this.module = moduleProvider;
+      this.linked = resolvedArrayPromise;
+      this.instantiated = this.modulePromise;
+    }
+  }
+
+  instantiate() {
+    if (this.instantiated) {
+      return this.instantiated;
+    }
+    return this.instantiated = new Promise(async (resolve, reject) => {
+      const jobsInGraph = new SafeSet();
+      let jobsReadyToInstantiate = 0;
+      // (this must be sync for counter to work)
+      const queueJob = (moduleJob) => {
+        if (jobsInGraph.has(moduleJob)) {
+          return;
+        }
+        jobsInGraph.add(moduleJob);
+        moduleJob.linked.then((dependencyJobs) => {
+          for (const dependencyJob of dependencyJobs) {
+            queueJob(dependencyJob);
+          }
+          checkComplete();
+        }, (e) => {
+          if (!this.hadError) {
+            this.error = e;
+            this.hadError = true;
+          }
+          checkComplete();
+        });
+      };
+      const checkComplete = () => {
+        if (++jobsReadyToInstantiate === jobsInGraph.size) {
+          // I believe we only throw once the whole tree is finished loading?
+          // or should the error bail early, leaving entire tree to still load?
+          if (this.hadError) {
+            reject(this.error);
+          } else {
+            try {
+              this.module.instantiate();
+              for (const dependencyJob of jobsInGraph) {
+                dependencyJob.instantiated = resolvedPromise;
+              }
+              resolve(this.module);
+            } catch (e) {
+              e.stack;
+              reject(e);
+            }
+          }
+        }
+      };
+      queueJob(this);
+    });
+  }
+
+  async run() {
+    const module = await this.instantiate();
+    try {
+      module.evaluate();
+    } catch (e) {
+      e.stack;
+      this.hadError = true;
+      this.error = e;
+      throw e;
+    }
+    return module;
+  }
+}
+Object.setPrototypeOf(ModuleJob.prototype, null);
+module.exports = ModuleJob;

lib/internal/loader/ModuleMap.js

@@ -0,0 +1,33 @@
+'use strict';
+
+const ModuleJob = require('internal/loader/ModuleJob');
+const { SafeMap } = require('internal/safe_globals');
+const debug = require('util').debuglog('esm');
+const errors = require('internal/errors');
+
+// Tracks the state of the loader-level module cache
+class ModuleMap extends SafeMap {
+  get(url) {
+    if (typeof url !== 'string') {
+      throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'url', 'string');
+    }
+    return super.get(url);
+  }
+  set(url, job) {
+    if (typeof url !== 'string') {
+      throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'url', 'string');
+    }
+    if (job instanceof ModuleJob !== true) {
+      throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'job', 'ModuleJob');
+    }
+    debug(`Storing ${url} in ModuleMap`);
+    return super.set(url, job);
+  }
+  has(url) {
+    if (typeof url !== 'string') {
+      throw new errors.TypeError('ERR_INVALID_ARG_TYPE', 'url', 'string');
+    }
+    return super.has(url);
+  }
+}
+module.exports = ModuleMap;