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: https://github.com/nodejs/node/pull/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>

46 files changed, 1578 insertions, 40 deletions

diff --git a/.eslintrc.yaml b/.eslintrc.yaml
index 36677b7cea0..b68d2657465 100644
--- a/.eslintrc.yaml
+++ b/.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
diff --git a/Makefile b/Makefile
index 67a568cfa54..547d93dc15b 100644
--- a/Makefile
+++ b/Makefile
@@ -150,7 +150,7 @@ 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
@@ -886,7 +886,7 @@ 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:
diff --git a/doc/api/esm.md b/doc/api/esm.md
new file mode 100644
index 00000000000..108fd763364
--- /dev/null
+++ b/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
diff --git a/lib/internal/bootstrap_node.js b/lib/internal/bootstrap_node.js
index 9d776674d2e..b43a262682c 100644
--- a/lib/internal/bootstrap_node.js
+++ b/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
diff --git a/lib/internal/errors.js b/lib/internal/errors.js
index 623e57d875e..10e5be8a440 100644
--- a/lib/internal/errors.js
+++ b/lib/internal/errors.js
@@ -229,6 +229,9 @@ 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_METHOD_NOT_IMPLEMENTED', 'The %s method is not implemented');
 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_MULTIPLE_CALLBACK', 'Callback called multiple times');
 E('ERR_NAPI_CONS_FUNCTION', 'Constructor must be a function');
 E('ERR_NAPI_CONS_PROTOTYPE_OBJECT', 'Constructor.prototype must be an object');
@@ -237,6 +240,7 @@ E('ERR_NO_ICU', '%s is not supported on Node.js compiled without ICU');
 E('ERR_NO_LONGER_SUPPORTED', '%s is no longer supported');
 E('ERR_OUTOFMEMORY', 'Out of memory');
 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_SERVER_ALREADY_LISTEN',
   'Listen method has been called more than once without closing.');
 E('ERR_SOCKET_ALREADY_BOUND', 'Socket is already bound');
diff --git a/lib/internal/loader/Loader.js b/lib/internal/loader/Loader.js
new file mode 100644
index 00000000000..a409d397f85
--- /dev/null
+++ b/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 = `${request.url}`;
+    let job;
+    if (this.moduleMap.has(url)) {
+      job = this.moduleMap.get(url);
+    } else {
+      job = new ModuleJob(this, request);
+      this.moduleMap.set(url, job);
+    }
+    const module = await job.run();
+    return getNamespaceOfModuleWrap(module);
+  }
+}
+Object.setPrototypeOf(Loader.prototype, null);
+module.exports = Loader;
diff --git a/lib/internal/loader/ModuleJob.js b/lib/internal/loader/ModuleJob.js
new file mode 100644
index 00000000000..db4cb6ae5c5
--- /dev/null
+++ b/lib/internal/loader/ModuleJob.js
@@ -0,0 +1,116 @@
+'use strict';
+
+const { SafeSet, SafePromise } = require('internal/safe_globals');
+const resolvedPromise = SafePromise.resolve();
+const resolvedArrayPromise = SafePromise.resolve([]);
+const { ModuleWrap } = require('internal/loader/ModuleWrap');
+
+const NOOP = () => { /* No-op */ };
+class ModuleJob {
+  /**
+   * @param {module: ModuleWrap?, compiled: Promise} moduleProvider
+   */
+  constructor(loader, moduleProvider, url) {
+    this.url = `${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;
diff --git a/lib/internal/loader/ModuleMap.js b/lib/internal/loader/ModuleMap.js
new file mode 100644
index 00000000000..aa238afbaed
--- /dev/null
+++ b/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;
diff --git a/lib/internal/loader/ModuleWrap.js b/lib/internal/loader/ModuleWrap.js
new file mode 100644
index 00000000000..4d35356ec24
--- /dev/null
+++ b/lib/internal/loader/ModuleWrap.js
@@ -0,0 +1,61 @@
+'use strict';
+
+const { ModuleWrap } = process.binding('module_wrap');
+const debug = require('util').debuglog('esm');
+const ArrayJoin = Function.call.bind(Array.prototype.join);
+const ArrayMap = Function.call.bind(Array.prototype.map);
+
+const getNamespaceOfModuleWrap = (m) => {
+  const tmp = new ModuleWrap('import * as _ from "";_;', '');
+  tmp.link(async () => m);
+  tmp.instantiate();
+  return tmp.evaluate();
+};
+
+const createDynamicModule = (exports, url = '', evaluate) => {
+  debug(
+    `creating ESM facade for ${url} with exports: ${ArrayJoin(exports, ', ')}`
+  );
+  const names = ArrayMap(exports, (name) => `${name}`);
+  // sanitized ESM for reflection purposes
+  const src = `export let executor;
+  ${ArrayJoin(ArrayMap(names, (name) => `export let $${name}`), ';\n')}
+  ;(() => [
+    fn => executor = fn,
+    { exports: { ${
+  ArrayJoin(ArrayMap(names, (name) => `${name}: {
+        get: () => $${name},
+        set: v => $${name} = v
+      }`), ',\n')
+} } }
+  ]);
+  `;
+  const reflectiveModule = new ModuleWrap(src, `cjs-facade:${url}`);
+  reflectiveModule.instantiate();
+  const [setExecutor, reflect] = reflectiveModule.evaluate()();
+  // public exposed ESM
+  const reexports = `import { executor,
+    ${ArrayMap(names, (name) => `$${name}`)}
+  } from "";
+  export {
+    ${ArrayJoin(ArrayMap(names, (name) => `$${name} as ${name}`), ', ')}
+  }
+  // add await to this later if top level await comes along
+  typeof executor === "function" ? executor() : void 0;`;
+  if (typeof evaluate === 'function') {
+    setExecutor(() => evaluate(reflect));
+  }
+  const runner = new ModuleWrap(reexports, `${url}`);
+  runner.link(async () => reflectiveModule);
+  runner.instantiate();
+  return {
+    module: runner,
+    reflect
+  };
+};
+
+module.exports = {
+  createDynamicModule,
+  getNamespaceOfModuleWrap,
+  ModuleWrap
+};
diff --git a/lib/internal/loader/resolveRequestUrl.js b/lib/internal/loader/resolveRequestUrl.js
new file mode 100644
index 00000000000..2245064bfe4
--- /dev/null
+++ b/lib/internal/loader/resolveRequestUrl.js
@@ -0,0 +1,104 @@
+'use strict';
+
+const { URL } = require('url');
+const internalCJSModule = require('internal/module');
+const internalURLModule = require('internal/url');
+const internalFS = require('internal/fs');
+const NativeModule = require('native_module');
+const { extname } = require('path');
+const { realpathSync } = require('fs');
+const preserveSymlinks = !!process.binding('config').preserveSymlinks;
+const {
+  ModuleWrap,
+  createDynamicModule
+} = require('internal/loader/ModuleWrap');
+const errors = require('internal/errors');
+
+const search = require('internal/loader/search');
+const asyncReadFile = require('util').promisify(require('fs').readFile);
+const debug = require('util').debuglog('esm');
+
+const realpathCache = new Map();
+
+class ModuleRequest {
+  constructor(url) {
+    this.url = url;
+  }
+}
+Object.setPrototypeOf(ModuleRequest.prototype, null);
+
+// Strategy for loading a standard JavaScript module
+class StandardModuleRequest extends ModuleRequest {
+  async createModule() {
+    const source = `${await asyncReadFile(this.url)}`;
+    debug(`Loading StandardModule ${this.url}`);
+    return new ModuleWrap(internalCJSModule.stripShebang(source),
+                          `${this.url}`);
+  }
+}
+
+// Strategy for loading a node-style CommonJS module
+class CJSModuleRequest extends ModuleRequest {
+  async createModule() {
+    const ctx = createDynamicModule(['default'], this.url, (reflect) => {
+      debug(`Loading CJSModule ${this.url.pathname}`);
+      const CJSModule = require('module');
+      const pathname = internalURLModule.getPathFromURL(this.url);
+      CJSModule._load(pathname);
+    });
+    this.finish = (module) => {
+      ctx.reflect.exports.default.set(module.exports);
+    };
+    return ctx.module;
+  }
+}
+
+// Strategy for loading a node builtin CommonJS module that isn't
+// through normal resolution
+class NativeModuleRequest extends CJSModuleRequest {
+  async createModule() {
+    const ctx = createDynamicModule(['default'], this.url, (reflect) => {
+      debug(`Loading NativeModule ${this.url.pathname}`);
+      const exports = require(this.url.pathname);
+      reflect.exports.default.set(exports);
+    });
+    return ctx.module;
+  }
+}
+
+const normalizeBaseURL = (baseURLOrString) => {
+  if (baseURLOrString instanceof URL) return baseURLOrString;
+  if (typeof baseURLOrString === 'string') return new URL(baseURLOrString);
+  return undefined;
+};
+
+const resolveRequestUrl = (baseURLOrString, specifier) => {
+  if (NativeModule.nonInternalExists(specifier)) {
+    return new NativeModuleRequest(new URL(`node:${specifier}`));
+  }
+
+  const baseURL = normalizeBaseURL(baseURLOrString);
+  let url = search(specifier, baseURL);
+
+  if (url.protocol !== 'file:') {
+    throw new errors.Error('ERR_INVALID_PROTOCOL', url.protocol, 'file:');
+  }
+
+  if (!preserveSymlinks) {
+    const real = realpathSync(internalURLModule.getPathFromURL(url), {
+      [internalFS.realpathCacheKey]: realpathCache
+    });
+    const old = url;
+    url = internalURLModule.getURLFromFilePath(real);
+    url.search = old.search;
+    url.hash = old.hash;
+  }
+
+  const ext = extname(url.pathname);
+  if (ext === '.mjs') {
+    return new StandardModuleRequest(url);
+  }
+
+  return new CJSModuleRequest(url);
+};
+module.exports = resolveRequestUrl;
diff --git a/lib/internal/loader/search.js b/lib/internal/loader/search.js
new file mode 100644
index 00000000000..f0ec34ae4e7
--- /dev/null
+++ b/lib/internal/loader/search.js
@@ -0,0 +1,33 @@
+'use strict';
+
+const { URL } = require('url');
+const CJSmodule = require('module');
+const errors = require('internal/errors');
+const { resolve } = process.binding('module_wrap');
+
+module.exports = (target, base) => {
+  target = `${target}`;
+  if (base === undefined) {
+    // We cannot search without a base.
+    throw new errors.Error('ERR_MISSING_MODULE', target);
+  }
+  base = `${base}`;
+  try {
+    return resolve(target, base);
+  } catch (e) {
+    e.stack; // cause V8 to generate stack before rethrow
+    let error = e;
+    try {
+      const questionedBase = new URL(base);
+      const tmpMod = new CJSmodule(questionedBase.pathname, null);
+      tmpMod.paths = CJSmodule._nodeModulePaths(
+        new URL('./', questionedBase).pathname);
+      const found = CJSmodule._resolveFilename(target, tmpMod);
+      error = new errors.Error('ERR_MODULE_RESOLUTION_LEGACY', target,
+                               base, found);
+    } catch (problemChecking) {
+      // ignore
+    }
+    throw error;
+  }
+};
diff --git a/lib/internal/safe_globals.js b/lib/internal/safe_globals.js
new file mode 100644
index 00000000000..ad58fa662b5
--- /dev/null
+++ b/lib/internal/safe_globals.js
@@ -0,0 +1,26 @@
+'use strict';
+
+const copyProps = (unsafe, safe) => {
+  for (const key of [...Object.getOwnPropertyNames(unsafe),
+                     ...Object.getOwnPropertySymbols(unsafe)
+  ]) {
+    if (!Object.getOwnPropertyDescriptor(safe, key)) {
+      Object.defineProperty(
+        safe,
+        key,
+        Object.getOwnPropertyDescriptor(unsafe, key));
+    }
+  }
+};
+const makeSafe = (unsafe, safe) => {
+  copyProps(unsafe.prototype, safe.prototype);
+  copyProps(unsafe, safe);
+  Object.setPrototypeOf(safe.prototype, null);
+  Object.freeze(safe.prototype);
+  Object.freeze(safe);
+  return safe;
+};
+
+exports.SafeMap = makeSafe(Map, class SafeMap extends Map {});
+exports.SafeSet = makeSafe(Set, class SafeSet extends Set {});
+exports.SafePromise = makeSafe(Promise, class SafePromise extends Promise {});
diff --git a/lib/internal/url.js b/lib/internal/url.js
index 95b5e9c5fd4..cf0271691a5 100644
--- a/lib/internal/url.js
+++ b/lib/internal/url.js
@@ -1377,6 +1377,12 @@ function getPathFromURL(path) {
   return isWindows ? getPathFromURLWin32(path) : getPathFromURLPosix(path);
 }
 
+function getURLFromFilePath(filepath) {
+  const tmp = new URL('file://');
+  tmp.pathname = filepath;
+  return tmp;
+}
+
 function NativeURL(ctx) {
   this[context] = ctx;
 }
@@ -1405,6 +1411,7 @@ setURLConstructor(constructUrl);
 module.exports = {
   toUSVString,
   getPathFromURL,
+  getURLFromFilePath,
   URL,
   URLSearchParams,
   domainToASCII,
diff --git a/lib/module.js b/lib/module.js
index 7bb8288f54f..0b87cf7480e 100644
--- a/lib/module.js
+++ b/lib/module.js
@@ -24,6 +24,7 @@
 const NativeModule = require('native_module');
 const util = require('util');
 const internalModule = require('internal/module');
+const { getURLFromFilePath } = require('internal/url');
 const vm = require('vm');
 const assert = require('assert').ok;
 const fs = require('fs');
@@ -32,6 +33,14 @@ const path = require('path');
 const internalModuleReadFile = process.binding('fs').internalModuleReadFile;
 const internalModuleStat = process.binding('fs').internalModuleStat;
 const preserveSymlinks = !!process.binding('config').preserveSymlinks;
+const experimentalModules = !!process.binding('config').experimentalModules;
+
+const errors = require('internal/errors');
+
+const Loader = require('internal/loader/Loader');
+const ModuleJob = require('internal/loader/ModuleJob');
+const { createDynamicModule } = require('internal/loader/ModuleWrap');
+const ESMLoader = new Loader();
 
 function stat(filename) {
   filename = path._makeLong(filename);
@@ -412,7 +421,36 @@ Module._load = function(request, parent, isMain) {
     debug('Module._load REQUEST %s parent: %s', request, parent.id);
   }
 
-  var filename = Module._resolveFilename(request, parent, isMain);
+  var filename = null;
+
+  if (isMain) {
+    let err;
+    try {
+      filename = Module._resolveFilename(request, parent, isMain);
+    } catch (e) {
+      // try to keep stack
+      e.stack;
+      err = e;
+    }
+    if (experimentalModules) {
+      if (filename === null || /\.mjs$/.test(filename)) {
+        try {
+          ESMLoader.import(request).catch((e) => {
+            console.error(e);
+            process.exit(1);
+          });
+          return;
+        } catch (e) {
+          // well, it isn't ESM
+        }
+      }
+    }
+    if (err) {
+      throw err;
+    }
+  } else {
+    filename = Module._resolveFilename(request, parent, isMain);
+  }
 
   var cachedModule = Module._cache[filename];
   if (cachedModule) {
@@ -482,6 +520,19 @@ Module.prototype.load = function(filename) {
   if (!Module._extensions[extension]) extension = '.js';
   Module._extensions[extension](this, filename);
   this.loaded = true;
+
+  if (experimentalModules) {
+    const url = getURLFromFilePath(filename);
+    if (ESMLoader.moduleMap.has(`${url}`) !== true) {
+      const ctx = createDynamicModule(['default'], url);
+      ctx.reflect.exports.default.set(this.exports);
+      ESMLoader.moduleMap.set(`${url}`,
+                              new ModuleJob(ESMLoader, ctx.module));
+    } else {
+      ESMLoader.moduleMap.get(`${url}`).moduleProvider.finish(
+        Module._cache[filename]);
+    }
+  }
 };
 
 
@@ -578,6 +629,11 @@ Module._extensions['.node'] = function(module, filename) {
   return process.dlopen(module, path._makeLong(filename));
 };
 
+if (experimentalModules) {
+  Module._extensions['.mjs'] = function(module, filename) {
+    throw new errors.Error('ERR_REQUIRE_ESM', filename);
+  };
+}
 
 // bootstrap main module.
 Module.runMain = function() {
diff --git a/node.gyp b/node.gyp
index 14acf375e12..79d9e0a68dc 100644
--- a/node.gyp
+++ b/node.gyp
@@ -91,6 +91,13 @@
       'lib/internal/http.js',
       'lib/internal/inspector_async_hook.js',
       'lib/internal/linkedlist.js',
+      'lib/internal/loader/Loader.js',
+      'lib/internal/loader/ModuleMap.js',
+      'lib/internal/loader/ModuleJob.js',
+      'lib/internal/loader/ModuleWrap.js',
+      'lib/internal/loader/resolveRequestUrl.js',
+      'lib/internal/loader/search.js',
+      'lib/internal/safe_globals.js',
       'lib/internal/net.js',
       'lib/internal/module.js',
       'lib/internal/os.js',
@@ -177,6 +184,7 @@
         'src/fs_event_wrap.cc',
         'src/handle_wrap.cc',
         'src/js_stream.cc',
+        'src/module_wrap.cc',
         'src/node.cc',
         'src/node_api.cc',
         'src/node_api.h',
@@ -230,6 +238,7 @@
         'src/env-inl.h',
         'src/handle_wrap.h',
         'src/js_stream.h',
+        'src/module_wrap.h',
         'src/node.h',
         'src/node_http2_core.h',
         'src/node_http2_core-inl.h',
diff --git a/src/module_wrap.cc b/src/module_wrap.cc
new file mode 100644
index 00000000000..05bbe04ef2e
--- /dev/null
+++ b/src/module_wrap.cc
@@ -0,0 +1,531 @@
+#include <algorithm>
+#include <limits.h>  // PATH_MAX
+#include <sys/stat.h>  // S_IFDIR
+#include "module_wrap.h"
+
+#include "env.h"
+#include "node_url.h"
+#include "util.h"
+#include "util-inl.h"
+
+namespace node {
+namespace loader {
+
+using node::url::URL;
+using node::url::URL_FLAGS_FAILED;
+using v8::Context;
+using v8::EscapableHandleScope;
+using v8::Function;
+using v8::FunctionCallbackInfo;
+using v8::FunctionTemplate;
+using v8::Integer;
+using v8::IntegrityLevel;
+using v8::Isolate;
+using v8::JSON;
+using v8::Local;
+using v8::MaybeLocal;
+using v8::Module;
+using v8::Object;
+using v8::Persistent;
+using v8::Promise;
+using v8::ScriptCompiler;
+using v8::ScriptOrigin;
+using v8::String;
+using v8::Value;
+
+static const char* EXTENSIONS[] = {".mjs", ".js", ".json", ".node"};
+std::map<int, std::vector<ModuleWrap*>*> ModuleWrap::module_map_;
+
+ModuleWrap::ModuleWrap(Environment* env,
+                       Local<Object> object,
+                       Local<Module> module,
+                       Local<String> url) : BaseObject(env, object) {
+  Isolate* iso = Isolate::GetCurrent();
+  module_.Reset(iso, module);
+  url_.Reset(iso, url);
+}
+
+ModuleWrap::~ModuleWrap() {
+  Local<Module> module = module_.Get(Isolate::GetCurrent());
+  std::vector<ModuleWrap*>* same_hash = module_map_[module->GetIdentityHash()];
+  auto it = std::find(same_hash->begin(), same_hash->end(), this);
+
+  if (it != same_hash->end()) {
+    same_hash->erase(it);
+  }
+
+  module_.Reset();
+}
+
+void ModuleWrap::New(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+
+  Isolate* iso = args.GetIsolate();
+
+  if (!args.IsConstructCall()) {
+    env->ThrowError("constructor must be called using new");
+    return;
+  }
+
+  if (args.Length() != 2) {
+    env->ThrowError("constructor must have exactly 2 arguments "
+                    "(string, string)");
+    return;
+  }
+
+  if (!args[0]->IsString()) {
+    env->ThrowError("first argument is not a string");
+    return;
+  }
+
+  auto source_text = args[0].As<String>();
+
+  if (!args[1]->IsString()) {
+    env->ThrowError("second argument is not a string");
+    return;
+  }
+
+  Local<String> url = args[1].As<String>();
+
+  Local<Module> mod;
+
+  // compile
+  {
+    ScriptOrigin origin(url,
+                        Integer::New(iso, 0),
+                        Integer::New(iso, 0),
+                        False(iso),
+                        Integer::New(iso, 0),
+                        FIXED_ONE_BYTE_STRING(iso, ""),
+                        False(iso),
+                        False(iso),
+                        True(iso));
+    ScriptCompiler::Source source(source_text, origin);
+    auto maybe_mod = ScriptCompiler::CompileModule(iso, &source);
+    if (maybe_mod.IsEmpty()) {
+        return;
+    }
+    mod = maybe_mod.ToLocalChecked();
+  }
+
+  auto that = args.This();
+  auto ctx = that->CreationContext();
+  auto url_str = FIXED_ONE_BYTE_STRING(iso, "url");
+
+  if (!that->Set(ctx, url_str, url).FromMaybe(false)) {
+    return;
+  }
+
+  ModuleWrap* obj =
+      new ModuleWrap(Environment::GetCurrent(ctx), that, mod, url);
+
+  if (ModuleWrap::module_map_.count(mod->GetIdentityHash()) == 0) {
+    ModuleWrap::module_map_[mod->GetIdentityHash()] =
+        new std::vector<ModuleWrap*>();
+  }
+
+  ModuleWrap::module_map_[mod->GetIdentityHash()]->push_back(obj);
+  Wrap(that, obj);
+
+  that->SetIntegrityLevel(ctx, IntegrityLevel::kFrozen);
+  args.GetReturnValue().Set(that);
+}
+
+void ModuleWrap::Link(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+  Isolate* iso = args.GetIsolate();
+  EscapableHandleScope handle_scope(iso);
+  if (!args[0]->IsFunction()) {
+    env->ThrowError("first argument is not a function");
+    return;
+  }
+
+  Local<Function> resolver_arg = args[0].As<Function>();
+
+  auto that = args.This();
+  ModuleWrap* obj = Unwrap<ModuleWrap>(that);
+  auto mod_context = that->CreationContext();
+  if (obj->linked_) return;
+  obj->linked_ = true;
+  Local<Module> mod(obj->module_.Get(iso));
+
+  // call the dependency resolve callbacks
+  for (int i = 0; i < mod->GetModuleRequestsLength(); i++) {
+    Local<String> specifier = mod->GetModuleRequest(i);
+    Utf8Value specifier_utf(env->isolate(), specifier);
+    std::string specifier_std(*specifier_utf, specifier_utf.length());
+
+    Local<Value> argv[] = {
+      specifier
+    };
+
+    MaybeLocal<Value> maybe_resolve_return_value =
+        resolver_arg->Call(mod_context, that, 1, argv);
+    if (maybe_resolve_return_value.IsEmpty()) {
+      return;
+    }
+    Local<Value> resolve_return_value =
+        maybe_resolve_return_value.ToLocalChecked();
+    if (!resolve_return_value->IsPromise()) {
+      env->ThrowError("linking error, expected resolver to return a promise");
+    }
+    Local<Promise> resolve_promise = resolve_return_value.As<Promise>();
+    obj->resolve_cache_[specifier_std] = new Persistent<Promise>();
+    obj->resolve_cache_[specifier_std]->Reset(iso, resolve_promise);
+  }
+
+  args.GetReturnValue().Set(handle_scope.Escape(that));
+}
+
+void ModuleWrap::Instantiate(const FunctionCallbackInfo<Value>& args) {
+  auto iso = args.GetIsolate();
+  auto that = args.This();
+  auto ctx = that->CreationContext();
+
+  ModuleWrap* obj = Unwrap<ModuleWrap>(that);
+  Local<Module> mod = obj->module_.Get(iso);
+  bool ok = mod->Instantiate(ctx, ModuleWrap::ResolveCallback);
+
+  // clear resolve cache on instantiate
+  obj->resolve_cache_.clear();
+
+  if (!ok) {
+    return;
+  }
+}
+
+void ModuleWrap::Evaluate(const FunctionCallbackInfo<Value>& args) {
+  auto iso = args.GetIsolate();
+  auto that = args.This();
+  auto ctx = that->CreationContext();
+  ModuleWrap* obj = Unwrap<ModuleWrap>(that);
+  auto result = obj->module_.Get(iso)->Evaluate(ctx);
+
+  if (result.IsEmpty()) {
+    return;
+  }
+
+  auto ret = result.ToLocalChecked();
+  args.GetReturnValue().Set(ret);
+}
+
+MaybeLocal<Module> ModuleWrap::ResolveCallback(Local<Context> context,
+                                               Local<String> specifier,
+                                               Local<Module> referrer) {
+  Environment* env = Environment::GetCurrent(context);
+  Isolate* iso = Isolate::GetCurrent();
+  if (ModuleWrap::module_map_.count(referrer->GetIdentityHash()) == 0) {
+    env->ThrowError("linking error, unknown module");
+    return MaybeLocal<Module>();
+  }
+
+  std::vector<ModuleWrap*>* possible_deps =
+      ModuleWrap::module_map_[referrer->GetIdentityHash()];
+  ModuleWrap* dependent = nullptr;
+
+  for (auto possible_dep : *possible_deps) {
+    if (possible_dep->module_ == referrer) {
+      dependent = possible_dep;
+    }
+  }
+
+  if (dependent == nullptr) {
+    env->ThrowError("linking error, null dep");
+    return MaybeLocal<Module>();
+  }
+
+  Utf8Value specifier_utf(env->isolate(), specifier);
+  std::string specifier_std(*specifier_utf, specifier_utf.length());
+
+  if (dependent->resolve_cache_.count(specifier_std) != 1) {
+    env->ThrowError("linking error, not in local cache");
+    return MaybeLocal<Module>();
+  }
+
+  Local<Promise> resolve_promise =
+      dependent->resolve_cache_[specifier_std]->Get(iso);
+
+  if (resolve_promise->State() != Promise::kFulfilled) {
+    env->ThrowError("linking error, dependency promises must be resolved on "
+                    "instantiate");
+    return MaybeLocal<Module>();
+  }
+
+  auto module_object = resolve_promise->Result().As<Object>();
+  if (module_object.IsEmpty() || !module_object->IsObject()) {
+    env->ThrowError("linking error, expected a valid module object from "
+                    "resolver");
+    return MaybeLocal<Module>();
+  }
+
+  ModuleWrap* mod;
+  ASSIGN_OR_RETURN_UNWRAP(&mod, module_object, MaybeLocal<Module>());
+  return mod->module_.Get(env->isolate());
+}
+
+namespace {
+
+URL __init_cwd() {
+  std::string specifier = "file://";
+#ifdef _WIN32
+  // MAX_PATH is in characters, not bytes. Make sure we have enough headroom.
+  char buf[MAX_PATH * 4];
+#else
+  char buf[PATH_MAX];
+#endif
+
+  size_t cwd_len = sizeof(buf);
+  int err = uv_cwd(buf, &cwd_len);
+  if (err) {
+    return URL("");
+  }
+  specifier += buf;
+  specifier += "/";
+  return URL(specifier);
+}
+static URL INITIAL_CWD(__init_cwd());
+inline bool is_relative_or_absolute_path(std::string specifier) {
+  auto len = specifier.length();
+  if (len <= 0) {
+    return false;
+  } else if (specifier[0] == '/') {
+    return true;
+  } else if (specifier[0] == '.') {
+    if (len == 1 || specifier[1] == '/') {
+      return true;
+    } else if (specifier[1] == '.') {
+      if (len == 2 || specifier[2] == '/') {
+        return true;
+      }
+    }
+  }
+  return false;
+}
+struct read_result {
+  bool had_error = false;
+  std::string source;
+} read_result;
+inline const struct read_result read_file(uv_file file) {
+  struct read_result ret;
+  std::string src;
+  uv_fs_t req;
+  void* base = malloc(4096);
+  if (base == nullptr) {
+    ret.had_error = true;
+    return ret;
+  }
+  uv_buf_t buf = uv_buf_init(static_cast<char*>(base), 4096);
+  uv_fs_read(uv_default_loop(), &req, file, &buf, 1, 0, nullptr);
+  while (req.result > 0) {
+    src += std::string(static_cast<const char*>(buf.base), req.result);
+    uv_fs_read(uv_default_loop(), &req, file, &buf, 1, src.length(), nullptr);
+  }
+  ret.source = src;
+  return ret;
+}
+struct file_check {
+  bool failed = true;
+  uv_file file;
+} file_check;
+inline const struct file_check check_file(URL search,
+                                          bool close = false,
+                                          bool allow_dir = false) {
+  struct file_check ret;
+  uv_fs_t fs_req;
+  std::string path = search.ToFilePath();
+  if (path.empty()) {
+    return ret;
+  }
+  uv_fs_open(nullptr, &fs_req, path.c_str(), O_RDONLY, 0, nullptr);
+  auto fd = fs_req.result;
+  if (fd < 0) {
+    return ret;
+  }
+  if (!allow_dir) {
+    uv_fs_fstat(nullptr, &fs_req, fd, nullptr);
+    if (fs_req.statbuf.st_mode & S_IFDIR) {
+      uv_fs_close(nullptr, &fs_req, fd, nullptr);
+      return ret;
+    }
+  }
+  ret.failed = false;
+  ret.file = fd;
+  if (close) uv_fs_close(nullptr, &fs_req, fd, nullptr);
+  return ret;
+}
+URL resolve_extensions(URL search, bool check_exact = true) {
+  if (check_exact) {
+    auto check = check_file(search, true);
+    if (!check.failed) {
+      return search;
+    }
+  }
+  for (auto extension : EXTENSIONS) {
+    URL guess(search.path() + extension, &search);
+    auto check = check_file(guess, true);
+    if (!check.failed) {
+      return guess;
+    }
+  }
+  return URL("");
+}
+inline URL resolve_index(URL search) {
+  return resolve_extensions(URL("index", &search), false);
+}
+URL resolve_main(URL search) {
+  URL pkg("package.json", &search);
+  auto check = check_file(pkg);
+  if (!check.failed) {
+    auto iso = Isolate::GetCurrent();
+    auto ctx = iso->GetCurrentContext();
+    auto read = read_file(check.file);
+    uv_fs_t fs_req;
+    // if we fail to close :-/
+    uv_fs_close(nullptr, &fs_req, check.file, nullptr);
+    if (read.had_error) return URL("");
+    std::string pkg_src = read.source;
+    Local<String> src =
+        String::NewFromUtf8(iso, pkg_src.c_str(),
+                            String::kNormalString, pkg_src.length());
+    if (src.IsEmpty()) return URL("");
+    auto maybe_pkg_json = JSON::Parse(ctx, src);
+    if (maybe_pkg_json.IsEmpty()) return URL("");
+    auto pkg_json_obj = maybe_pkg_json.ToLocalChecked().As<Object>();
+    if (!pkg_json_obj->IsObject()) return URL("");
+    auto maybe_pkg_main = pkg_json_obj->Get(
+        ctx, FIXED_ONE_BYTE_STRING(iso, "main"));
+    if (maybe_pkg_main.IsEmpty()) return URL("");
+    auto pkg_main_str = maybe_pkg_main.ToLocalChecked().As<String>();
+    if (!pkg_main_str->IsString()) return URL("");
+    Utf8Value main_utf8(iso, pkg_main_str);
+    std::string main_std(*main_utf8, main_utf8.length());
+    if (!is_relative_or_absolute_path(main_std)) {
+      main_std.insert(0, "./");
+    }
+    return Resolve(main_std, &search);
+  }
+  return URL("");
+}
+URL resolve_module(std::string specifier, URL* base) {
+  URL parent(".", base);
+  URL dir("");
+  do {
+    dir = parent;
+    auto check = Resolve("./node_modules/" + specifier, &dir, true);
+    if (!(check.flags() & URL_FLAGS_FAILED)) {
+      const auto limit = specifier.find('/');
+      const auto spec_len = limit == std::string::npos ?
+                                specifier.length() :
+                                limit + 1;
+      std::string chroot =
+          dir.path() + "node_modules/" + specifier.substr(0, spec_len);
+      if (check.path().substr(0, chroot.length()) != chroot) {
+        return URL("");
+      }
+      return check;
+    } else {
+      // TODO(bmeck) PREVENT FALLTHROUGH
+    }
+    parent = URL("..", &dir);
+  } while (parent.path() != dir.path());
+  return URL("");
+}
+
+URL resolve_directory(URL search, bool read_pkg_json) {
+  if (read_pkg_json) {
+    auto main = resolve_main(search);
+    if (!(main.flags() & URL_FLAGS_FAILED)) return main;
+  }
+  return resolve_index(search);
+}
+
+}  // anonymous namespace
+
+
+URL Resolve(std::string specifier, URL* base, bool read_pkg_json) {
+  URL pure_url(specifier);
+  if (!(pure_url.flags() & URL_FLAGS_FAILED)) {
+    return pure_url;
+  }
+  if (specifier.length() == 0) {
+    return URL("");
+  }
+  if (is_relative_or_absolute_path(specifier)) {
+    URL resolved(specifier, base);
+    auto file = resolve_extensions(resolved);
+    if (!(file.flags() & URL_FLAGS_FAILED)) return file;
+    if (specifier.back() != '/') {
+      resolved = URL(specifier + "/", base);
+    }
+    return resolve_directory(resolved, read_pkg_json);
+  } else {
+    return resolve_module(specifier, base);
+  }
+  return URL("");
+}
+
+void ModuleWrap::Resolve(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+
+  if (args.IsConstructCall()) {
+    env->ThrowError("resolve() must not be called as a constructor");
+    return;
+  }
+  if (args.Length() != 2) {
+    env->ThrowError("resolve must have exactly 2 arguments (string, string)");
+    return;
+  }
+
+  if (!args[0]->IsString()) {
+    env->ThrowError("first argument is not a string");
+    return;
+  }
+  Utf8Value specifier_utf(env->isolate(), args[0]);
+
+  if (!args[1]->IsString()) {
+    env->ThrowError("second argument is not a string");
+    return;
+  }
+  Utf8Value url_utf(env->isolate(), args[1]);
+  URL url(*url_utf, url_utf.length());
+
+  if (url.flags() & URL_FLAGS_FAILED) {
+    env->ThrowError("second argument is not a URL string");
+    return;
+  }
+
+  URL result = node::loader::Resolve(*specifier_utf, &url, true);
+  if (result.flags() & URL_FLAGS_FAILED) {
+    std::string msg = "module ";
+    msg += *specifier_utf;
+    msg += " not found";
+    env->ThrowError(msg.c_str());
+    return;
+  }
+
+  args.GetReturnValue().Set(result.ToObject(env));
+}
+
+void ModuleWrap::Initialize(Local<Object> target,
+                           Local<Value> unused,
+                           Local<Context> context) {
+  Environment* env = Environment::GetCurrent(context);
+  Isolate* isolate = env->isolate();
+
+  Local<FunctionTemplate> tpl = env->NewFunctionTemplate(New);
+  tpl->SetClassName(FIXED_ONE_BYTE_STRING(isolate, "ModuleWrap"));
+  tpl->InstanceTemplate()->SetInternalFieldCount(1);
+
+  env->SetProtoMethod(tpl, "link", Link);
+  env->SetProtoMethod(tpl, "instantiate", Instantiate);
+  env->SetProtoMethod(tpl, "evaluate", Evaluate);
+
+  target->Set(FIXED_ONE_BYTE_STRING(isolate, "ModuleWrap"), tpl->GetFunction());
+  env->SetMethod(target, "resolve", node::loader::ModuleWrap::Resolve);
+}
+
+}  // namespace loader
+}  // namespace node
+
+NODE_MODULE_CONTEXT_AWARE_BUILTIN(module_wrap,
+                                  node::loader::ModuleWrap::Initialize)
diff --git a/src/module_wrap.h b/src/module_wrap.h
new file mode 100644
index 00000000000..c669834c6f3
--- /dev/null
+++ b/src/module_wrap.h
@@ -0,0 +1,58 @@
+#ifndef SRC_MODULE_WRAP_H_
+#define SRC_MODULE_WRAP_H_
+
+#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#include <map>
+#include <string>
+#include <vector>
+#include "node_url.h"
+#include "base-object.h"
+#include "base-object-inl.h"
+
+namespace node {
+namespace loader {
+
+node::url::URL Resolve(std::string specifier, node::url::URL* base,
+                       bool read_pkg_json = false);
+
+class ModuleWrap : public BaseObject {
+ public:
+  static const std::string EXTENSIONS[];
+  static void Initialize(v8::Local<v8::Object> target,
+                         v8::Local<v8::Value> unused,
+                         v8::Local<v8::Context> context);
+
+ private:
+  ModuleWrap(node::Environment* env,
+             v8::Local<v8::Object> object,
+             v8::Local<v8::Module> module,
+             v8::Local<v8::String> url);
+  ~ModuleWrap();
+
+  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
+  static void Link(const v8::FunctionCallbackInfo<v8::Value>& args);
+  static void Instantiate(const v8::FunctionCallbackInfo<v8::Value>& args);
+  static void Evaluate(const v8::FunctionCallbackInfo<v8::Value>& args);
+  static void GetUrl(v8::Local<v8::String> property,
+                     const v8::PropertyCallbackInfo<v8::Value>& info);
+  static void Resolve(const v8::FunctionCallbackInfo<v8::Value>& args);
+  static v8::MaybeLocal<v8::Module> ResolveCallback(
+      v8::Local<v8::Context> context,
+      v8::Local<v8::String> specifier,
+      v8::Local<v8::Module> referrer);
+
+  v8::Persistent<v8::Module> module_;
+  v8::Persistent<v8::String> url_;
+  bool linked_ = false;
+  std::map<std::string, v8::Persistent<v8::Promise>*> resolve_cache_;
+
+  static std::map<int, std::vector<ModuleWrap*>*> module_map_;
+};
+
+}  // namespace loader
+}  // namespace node
+
+#endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#endif  // SRC_MODULE_WRAP_H_
diff --git a/src/node.cc b/src/node.cc
index bdff4527c67..5cd0ffc29db 100644
--- a/src/node.cc
+++ b/src/node.cc
@@ -225,6 +225,11 @@ bool trace_warnings = false;
 // that is used by lib/module.js
 bool config_preserve_symlinks = false;
 
+// Set in node.cc by ParseArgs when --experimental-modules is used.
+// Used in node_config.cc to set a constant on process.binding('config')
+// that is used by lib/module.js
+bool config_experimental_modules = false;
+
 // Set by ParseArgs when --pending-deprecation or NODE_PENDING_DEPRECATION
 // is used.
 bool config_pending_deprecation = false;
@@ -3711,6 +3716,7 @@ static void PrintHelp() {
          "                             note: linked-in ICU data is present\n"
 #endif
          "  --preserve-symlinks        preserve symbolic links when resolving\n"
+         "  --experimental-modules     experimental ES Module support\n"
          "                             and caching modules\n"
 #endif
          "\n"
@@ -3947,6 +3953,8 @@ static void ParseArgs(int* argc,
       Revert(cve);
     } else if (strcmp(arg, "--preserve-symlinks") == 0) {
       config_preserve_symlinks = true;
+    } else if (strcmp(arg, "--experimental-modules") == 0) {
+      config_experimental_modules = true;
     } else if (strcmp(arg, "--prof-process") == 0) {
       prof_process = true;
       short_circuit = true;
diff --git a/src/node_config.cc b/src/node_config.cc
index 87110dd8c64..2f45a5e9712 100644
--- a/src/node_config.cc
+++ b/src/node_config.cc
@@ -65,6 +65,9 @@ static void InitConfig(Local<Object> target,
   if (config_preserve_symlinks)
     READONLY_BOOLEAN_PROPERTY("preserveSymlinks");
 
+  if (config_experimental_modules)
+    READONLY_BOOLEAN_PROPERTY("experimentalModules");
+
   if (config_pending_deprecation)
     READONLY_BOOLEAN_PROPERTY("pendingDeprecation");
 
diff --git a/src/node_internals.h b/src/node_internals.h
index 1e099325a35..a241e671edd 100644
--- a/src/node_internals.h
+++ b/src/node_internals.h
@@ -86,6 +86,10 @@ extern bool config_preserve_symlinks;
 
 // Set in node.cc by ParseArgs when --expose-http2 is used.
 extern bool config_expose_http2;
+// Set in node.cc by ParseArgs when --experimental-modules is used.
+// Used in node_config.cc to set a constant on process.binding('config')
+// that is used by lib/module.js
+extern bool config_experimental_modules;
 
 // Set in node.cc by ParseArgs when --expose-internals or --expose_internals is
 // used.
diff --git a/src/node_url.cc b/src/node_url.cc
index dd3da1133eb..f8adc7d7af5 100644
--- a/src/node_url.cc
+++ b/src/node_url.cc
@@ -2080,6 +2080,69 @@ static void DomainToUnicode(const FunctionCallbackInfo<Value>& args) {
                           v8::NewStringType::kNormal).ToLocalChecked());
 }
 
+std::string URL::ToFilePath() {
+  if (context_.scheme != "file:") {
+    return "";
+  }
+
+#ifdef _WIN32
+  const char* slash = "\\";
+  auto is_slash = [] (char ch) {
+    return ch == '/' || ch == '\\';
+  };
+#else
+  const char* slash = "/";
+  auto is_slash = [] (char ch) {
+    return ch == '/';
+  };
+  if ((context_.flags & URL_FLAGS_HAS_HOST) &&
+      context_.host.length() > 0) {
+    return "";
+  }
+#endif
+  std::string decoded_path;
+  for (std::string& part : context_.path) {
+    std::string decoded;
+    PercentDecode(part.c_str(), part.length(), &decoded);
+    for (char& ch : decoded) {
+      if (is_slash(ch)) {
+        return "";
+      }
+    }
+    decoded_path += slash + decoded;
+  }
+
+#ifdef _WIN32
+  // TODO(TimothyGu): Use "\\?\" long paths on Windows.
+
+  // If hostname is set, then we have a UNC path. Pass the hostname through
+  // ToUnicode just in case it is an IDN using punycode encoding. We do not
+  // need to worry about percent encoding because the URL parser will have
+  // already taken care of that for us. Note that this only causes IDNs with an
+  // appropriate `xn--` prefix to be decoded.
+  if ((context_.flags & URL_FLAGS_HAS_HOST) &&
+      context_.host.length() > 0) {
+    std::string unicode_host;
+    if (!ToUnicode(&context_.host, &unicode_host)) {
+      return "";
+    }
+    return "\\\\" + unicode_host + decoded_path;
+  }
+  // Otherwise, it's a local path that requires a drive letter.
+  if (decoded_path.length() < 3) {
+    return "";
+  }
+  if (decoded_path[2] != ':' ||
+      !IsASCIIAlpha(decoded_path[1])) {
+    return "";
+  }
+  // Strip out the leading '\'.
+  return decoded_path.substr(1);
+#else
+  return decoded_path;
+#endif
+}
+
 // This function works by calling out to a JS function that creates and
 // returns the JS URL object. Be mindful of the JS<->Native boundary
 // crossing that is required.
diff --git a/src/node_url.h b/src/node_url.h
index 72ac366ec13..cb7bdca7f2c 100644
--- a/src/node_url.h
+++ b/src/node_url.h
@@ -163,6 +163,10 @@ class URL {
     return ret;
   }
 
+  // Get the path of the file: URL in a format consumable by native file system
+  // APIs. Returns an empty string if something went wrong.
+  std::string ToFilePath();
+
   const Local<Value> ToObject(Environment* env) const;
 
  private:
diff --git a/test/cctest/test_url.cc b/test/cctest/test_url.cc
index 2cede1a8a3d..0b80d44caad 100644
--- a/test/cctest/test_url.cc
+++ b/test/cctest/test_url.cc
@@ -79,3 +79,28 @@ TEST_F(URLTest, Base3) {
   EXPECT_EQ(simple.host(), "example.org");
   EXPECT_EQ(simple.path(), "/baz");
 }
+
+TEST_F(URLTest, ToFilePath) {
+#define T(url, path) EXPECT_EQ(path, URL(url).ToFilePath())
+  T("http://example.org/foo/bar", "");
+
+#ifdef _WIN32
+  T("file:///C:/Program%20Files/", "C:\\Program Files\\");
+  T("file:///C:/a/b/c?query#fragment", "C:\\a\\b\\c");
+  T("file://host/path/a/b/c?query#fragment", "\\\\host\\path\\a\\b\\c");
+  T("file://xn--weird-prdj8vva.com/host/a", "\\\\wͪ͊eiͬ͋rd.com\\host\\a");
+  T("file:///C:/a%2Fb", "");
+  T("file:///", "");
+  T("file:///home", "");
+#else
+  T("file:///", "/");
+  T("file:///home/user?query#fragment", "/home/user");
+  T("file:///home/user/?query#fragment", "/home/user/");
+  T("file:///home/user/%20space", "/home/user/ space");
+  T("file:///home/us%5Cer", "/home/us\\er");
+  T("file:///home/us%2Fer", "");
+  T("file://host/path", "");
+#endif
+
+#undef T
+}
diff --git a/test/es-module/es-module.status b/test/es-module/es-module.status
new file mode 100644
index 00000000000..343e622ca02
--- /dev/null
+++ b/test/es-module/es-module.status
@@ -0,0 +1,7 @@
+prefix parallel
+
+# To mark a test as flaky, list the test name in the appropriate section
+# below, without ".js", followed by ": PASS,FLAKY". Example:
+# sample-test                        : PASS,FLAKY
+
+[true] # This section applies to all platforms
diff --git a/test/es-module/esm-snapshot-mutator.js b/test/es-module/esm-snapshot-mutator.js
new file mode 100644
index 00000000000..a0dfa0c28a9
--- /dev/null
+++ b/test/es-module/esm-snapshot-mutator.js
@@ -0,0 +1,5 @@
+/* eslint-disable required-modules */
+'use strict';
+const shouldSnapshotFilePath = require.resolve('./esm-snapshot.js');
+require('./esm-snapshot.js');
+require.cache[shouldSnapshotFilePath].exports++;
diff --git a/test/es-module/esm-snapshot.js b/test/es-module/esm-snapshot.js
new file mode 100644
index 00000000000..2c3c3a459a7
--- /dev/null
+++ b/test/es-module/esm-snapshot.js
@@ -0,0 +1,3 @@
+/* eslint-disable required-modules */
+'use strict';
+module.exports = 1;
diff --git a/test/es-module/test-esm-basic-imports.mjs b/test/es-module/test-esm-basic-imports.mjs
new file mode 100644
index 00000000000..23989bddd5b
--- /dev/null
+++ b/test/es-module/test-esm-basic-imports.mjs
@@ -0,0 +1,8 @@
+// Flags: --experimental-modules
+import '../common';
+import assert from 'assert';
+import ok from './test-esm-ok.mjs';
+import okShebang from './test-esm-shebang.mjs';
+
+assert(ok);
+assert(okShebang);
diff --git a/test/es-module/test-esm-encoded-path-native.js b/test/es-module/test-esm-encoded-path-native.js
new file mode 100644
index 00000000000..f32297efdb9
--- /dev/null
+++ b/test/es-module/test-esm-encoded-path-native.js
@@ -0,0 +1,10 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const { spawn } = require('child_process');
+
+const native = `${common.fixturesDir}/es-module-url/native.mjs`;
+const child = spawn(process.execPath, ['--experimental-modules', native]);
+child.on('exit', (code) => {
+  assert.strictEqual(code, 1);
+});
diff --git a/test/es-module/test-esm-encoded-path.mjs b/test/es-module/test-esm-encoded-path.mjs
new file mode 100644
index 00000000000..2c6e145927a
--- /dev/null
+++ b/test/es-module/test-esm-encoded-path.mjs
@@ -0,0 +1,7 @@
+// Flags: --experimental-modules
+import '../common';
+import assert from 'assert';
+// ./test-esm-ok.mjs
+import ok from './test-%65%73%6d-ok.mjs';
+
+assert(ok);
diff --git a/test/es-module/test-esm-forbidden-globals.mjs b/test/es-module/test-esm-forbidden-globals.mjs
new file mode 100644
index 00000000000..d3e92b9238a
--- /dev/null
+++ b/test/es-module/test-esm-forbidden-globals.mjs
@@ -0,0 +1,24 @@
+// Flags: --experimental-modules
+/* eslint-disable required-modules */
+
+if (typeof arguments !== 'undefined') {
+  throw new Error('not an ESM');
+}
+if (typeof this !== 'undefined') {
+  throw new Error('not an ESM');
+}
+if (typeof exports !== 'undefined') {
+  throw new Error('not an ESM');
+}
+if (typeof require !== 'undefined') {
+  throw new Error('not an ESM');
+}
+if (typeof module !== 'undefined') {
+  throw new Error('not an ESM');
+}
+if (typeof __filename !== 'undefined') {
+  throw new Error('not an ESM');
+}
+if (typeof __dirname !== 'undefined') {
+  throw new Error('not an ESM');
+}
diff --git a/test/es-module/test-esm-namespace.mjs b/test/es-module/test-esm-namespace.mjs
new file mode 100644
index 00000000000..72b7fed4b33
--- /dev/null
+++ b/test/es-module/test-esm-namespace.mjs
@@ -0,0 +1,7 @@
+// Flags: --experimental-modules
+/* eslint-disable required-modules */
+
+import * as fs from 'fs';
+import assert from 'assert';
+
+assert.deepStrictEqual(Object.keys(fs), ['default']);
diff --git a/test/es-module/test-esm-ok.mjs b/test/es-module/test-esm-ok.mjs
new file mode 100644
index 00000000000..6712e1ab7df
--- /dev/null
+++ b/test/es-module/test-esm-ok.mjs
@@ -0,0 +1,5 @@
+// Flags: --experimental-modules
+/* eslint-disable required-modules */
+
+const isJs = true;
+export default isJs;
diff --git a/test/es-module/test-esm-pkg-over-ext.mjs b/test/es-module/test-esm-pkg-over-ext.mjs
new file mode 100644
index 00000000000..7e47c4c3269
--- /dev/null
+++ b/test/es-module/test-esm-pkg-over-ext.mjs
@@ -0,0 +1,8 @@
+// Flags: --experimental-modules
+/* eslint-disable required-modules */
+
+import resolved from '../fixtures/module-pkg-over-ext/inner';
+import expected from '../fixtures/module-pkg-over-ext/inner/package.json';
+import assert from 'assert';
+
+assert.strictEqual(resolved, expected);
diff --git a/test/es-module/test-esm-preserve-symlinks.js b/test/es-module/test-esm-preserve-symlinks.js
new file mode 100644
index 00000000000..eea5bf061b2
--- /dev/null
+++ b/test/es-module/test-esm-preserve-symlinks.js
@@ -0,0 +1,38 @@
+// Flags: --experimental-modules
+'use strict';
+
+const common = require('../common');
+const { spawn } = require('child_process');
+const assert = require('assert');
+const path = require('path');
+const fs = require('fs');
+
+common.refreshTmpDir();
+const tmpDir = common.tmpDir;
+
+const entry = path.join(tmpDir, 'entry.js');
+const real = path.join(tmpDir, 'real.js');
+const link_absolute_path = path.join(tmpDir, 'link.js');
+
+fs.writeFileSync(entry, `
+const assert = require('assert');
+global.x = 0;
+require('./real.js');
+assert.strictEqual(x, 1);
+require('./link.js');
+assert.strictEqual(x, 2);
+`);
+fs.writeFileSync(real, 'x++;');
+
+try {
+  fs.symlinkSync(real, link_absolute_path);
+} catch (err) {
+  if (err.code !== 'EPERM') throw err;
+  common.skip('insufficient privileges for symlinks');
+}
+
+spawn(process.execPath,
+      ['--experimental-modules', '--preserve-symlinks', entry],
+      { stdio: 'inherit' }).on('exit', (code) => {
+  assert.strictEqual(code, 0);
+});
diff --git a/test/es-module/test-esm-require-cache.mjs b/test/es-module/test-esm-require-cache.mjs
new file mode 100644
index 00000000000..ff32cde36ff
--- /dev/null
+++ b/test/es-module/test-esm-require-cache.mjs
@@ -0,0 +1,7 @@
+// Flags: --experimental-modules
+import '../common';
+import '../fixtures/es-module-require-cache/preload.js';
+import '../fixtures/es-module-require-cache/counter.js';
+import assert from 'assert';
+assert.strictEqual(global.counter, 1);
+delete global.counter;
diff --git a/test/es-module/test-esm-shebang.mjs b/test/es-module/test-esm-shebang.mjs
new file mode 100644
index 00000000000..43cc0f8367d
--- /dev/null
+++ b/test/es-module/test-esm-shebang.mjs
@@ -0,0 +1,6 @@
+#! }]) // isn't js
+// Flags: --experimental-modules
+/* eslint-disable required-modules */
+
+const isJs = true;
+export default isJs;
diff --git a/test/es-module/test-esm-snapshot.mjs b/test/es-module/test-esm-snapshot.mjs
new file mode 100644
index 00000000000..89034f56681
--- /dev/null
+++ b/test/es-module/test-esm-snapshot.mjs
@@ -0,0 +1,7 @@
+// Flags: --experimental-modules
+/* eslint-disable required-modules */
+import './esm-snapshot-mutator';
+import one from './esm-snapshot';
+import assert from 'assert';
+
+assert.strictEqual(one, 1);
diff --git a/test/es-module/test-esm-symlink.js b/test/es-module/test-esm-symlink.js
new file mode 100644
index 00000000000..3b7d689bf8f
--- /dev/null
+++ b/test/es-module/test-esm-symlink.js
@@ -0,0 +1,48 @@
+'use strict';
+
+const common = require('../common');
+const { spawn } = require('child_process');
+const assert = require('assert');
+const path = require('path');
+const fs = require('fs');
+
+common.refreshTmpDir();
+const tmpDir = common.tmpDir;
+
+const entry = path.join(tmpDir, 'entry.mjs');
+const real = path.join(tmpDir, 'index.mjs');
+const link_absolute_path = path.join(tmpDir, 'absolute');
+const link_relative_path = path.join(tmpDir, 'relative');
+const link_ignore_extension = path.join(tmpDir,
+                                        'ignore_extension.json');
+const link_directory = path.join(tmpDir, 'directory');
+
+fs.writeFileSync(real, 'export default [];');
+fs.writeFileSync(entry, `
+import assert from 'assert';
+import real from './index.mjs';
+import absolute from './absolute';
+import relative from './relative';
+import ignoreExtension from './ignore_extension.json';
+import directory from './directory';
+
+assert.strictEqual(absolute, real);
+assert.strictEqual(relative, real);
+assert.strictEqual(ignoreExtension, real);
+assert.strictEqual(directory, real);
+`);
+
+try {
+  fs.symlinkSync(real, link_absolute_path);
+  fs.symlinkSync(path.basename(real), link_relative_path);
+  fs.symlinkSync(real, link_ignore_extension);
+  fs.symlinkSync(path.dirname(real), link_directory);
+} catch (err) {
+  if (err.code !== 'EPERM') throw err;
+  common.skip('insufficient privileges for symlinks');
+}
+
+spawn(process.execPath, ['--experimental-modules', entry],
+      { stdio: 'inherit' }).on('exit', (code) => {
+  assert.strictEqual(code, 0);
+});
diff --git a/test/es-module/testcfg.py b/test/es-module/testcfg.py
new file mode 100644
index 00000000000..0d8dfeed463
--- /dev/null
+++ b/test/es-module/testcfg.py
@@ -0,0 +1,6 @@
+import sys, os
+sys.path.append(os.path.join(os.path.dirname(__file__), '..'))
+import testpy
+
+def GetConfiguration(context, root):
+  return testpy.SimpleTestConfiguration(context, root, 'es-module')
diff --git a/test/fixtures/es-module-require-cache/counter.js b/test/fixtures/es-module-require-cache/counter.js
new file mode 100644
index 00000000000..2640d3e372d
--- /dev/null
+++ b/test/fixtures/es-module-require-cache/counter.js
@@ -0,0 +1,2 @@
+global.counter = global.counter || 0;
+global.counter++;
\ No newline at end of file
diff --git a/test/fixtures/es-module-require-cache/preload.js b/test/fixtures/es-module-require-cache/preload.js
new file mode 100644
index 00000000000..6090dc0d582
--- /dev/null
+++ b/test/fixtures/es-module-require-cache/preload.js
@@ -0,0 +1 @@
+require('./counter');
\ No newline at end of file
diff --git a/test/fixtures/es-module-url/empty.js b/test/fixtures/es-module-url/empty.js
new file mode 100644
index 00000000000..e69de29bb2d
--- /dev/null
+++ b/test/fixtures/es-module-url/empty.js
diff --git a/test/fixtures/es-module-url/native.mjs b/test/fixtures/es-module-url/native.mjs
new file mode 100644
index 00000000000..c8831f9bfee
--- /dev/null
+++ b/test/fixtures/es-module-url/native.mjs
@@ -0,0 +1,2 @@
+// path
+import 'p%61th';
diff --git a/test/testpy/__init__.py b/test/testpy/__init__.py
index 37e5ac710bc..f113c1253a4 100644
--- a/test/testpy/__init__.py
+++ b/test/testpy/__init__.py
@@ -27,7 +27,7 @@
 
 import test
 import os
-from os.path import join, dirname, exists
+from os.path import join, dirname, exists, splitext
 import re
 import ast
 
@@ -109,18 +109,17 @@ class SimpleTestConfiguration(test.TestConfiguration):
       self.additional_flags = []
 
   def Ls(self, path):
-    def SelectTest(name):
-      return name.startswith('test-') and name.endswith('.js')
-    return [f[:-3] for f in os.listdir(path) if SelectTest(f)]
+    return [f for f in os.listdir(path) if re.match('^test-.*\.m?js$', f)]
 
   def ListTests(self, current_path, path, arch, mode):
     all_tests = [current_path + [t] for t in self.Ls(join(self.root))]
     result = []
     for test in all_tests:
       if self.Contains(path, test):
-        file_path = join(self.root, reduce(join, test[1:], "") + ".js")
-        result.append(SimpleTestCase(test, file_path, arch, mode, self.context,
-                                     self, self.additional_flags))
+        file_path = join(self.root, reduce(join, test[1:], ""))
+        test_name = test[:-1] + [splitext(test[-1])[0]]
+        result.append(SimpleTestCase(test_name, file_path, arch, mode,
+                                     self.context, self, self.additional_flags))
     return result
 
   def GetBuildRequirements(self):
diff --git a/tools/eslint-rules/required-modules.js b/tools/eslint-rules/required-modules.js
index 47ade5cd9f9..948c46c036d 100644
--- a/tools/eslint-rules/required-modules.js
+++ b/tools/eslint-rules/required-modules.js
@@ -13,6 +13,7 @@ const path = require('path');
 module.exports = function(context) {
   // trim required module names
   var requiredModules = context.options;
+  const isESM = context.parserOptions.sourceType === 'module';
 
   const foundModules = [];
 
@@ -40,38 +41,34 @@ module.exports = function(context) {
   }
 
   /**
+   * Function to check if the path is a required module and return its name.
+   * @param {String} str The path to check
+   * @returns {undefined|String} required module name or undefined
+   */
+  function getRequiredModuleName(str) {
+    var value = path.basename(str);
+
+    // check if value is in required modules array
+    return requiredModules.indexOf(value) !== -1 ? value : undefined;
+  }
+
+  /**
    * Function to check if a node has an argument that is a required module and
    * return its name.
    * @param {ASTNode} node The node to check
    * @returns {undefined|String} required module name or undefined
    */
-  function getRequiredModuleName(node) {
-    var moduleName;
-
+  function getRequiredModuleNameFromCall(node) {
     // node has arguments and first argument is string
     if (node.arguments.length && isString(node.arguments[0])) {
-      var argValue = path.basename(node.arguments[0].value.trim());
-
-      // check if value is in required modules array
-      if (requiredModules.indexOf(argValue) !== -1) {
-        moduleName = argValue;
-      }
+      return getRequiredModuleName(node.arguments[0].value.trim());
     }
 
-    return moduleName;
+    return undefined;
   }
 
-  return {
-    'CallExpression': function(node) {
-      if (isRequireCall(node)) {
-        var requiredModuleName = getRequiredModuleName(node);
-
-        if (requiredModuleName) {
-          foundModules.push(requiredModuleName);
-        }
-      }
-    },
-    'Program:exit': function(node) {
+  const rules = {
+    'Program:exit'(node) {
       if (foundModules.length < requiredModules.length) {
         var missingModules = requiredModules.filter(
           function(module) {
@@ -88,6 +85,27 @@ module.exports = function(context) {
       }
     }
   };
+
+  if (isESM) {
+    rules.ImportDeclaration = (node) => {
+      var requiredModuleName = getRequiredModuleName(node.source.value);
+      if (requiredModuleName) {
+        foundModules.push(requiredModuleName);
+      }
+    };
+  } else {
+    rules.CallExpression = (node) => {
+      if (isRequireCall(node)) {
+        var requiredModuleName = getRequiredModuleNameFromCall(node);
+
+        if (requiredModuleName) {
+          foundModules.push(requiredModuleName);
+        }
+      }
+    };
+  }
+
+  return rules;
 };
 
 module.exports.schema = {
diff --git a/tools/test.py b/tools/test.py
index 5a50c7f2e6c..6839f4e1b2a 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -279,9 +279,7 @@ class TapProgressIndicator(SimpleProgressIndicator):
     # hard to decipher what test is running when only the filename is printed.
     prefix = abspath(join(dirname(__file__), '../test')) + os.sep
     command = output.command[-1]
-    if command.endswith('.js'): command = command[:-3]
-    if command.startswith(prefix): command = command[len(prefix):]
-    command = command.replace('\\', '/')
+    command = NormalizePath(command, prefix)
 
     if output.UnexpectedOutput():
       status_line = 'not ok %i %s' % (self._done, command)
@@ -352,9 +350,7 @@ class DeoptsCheckProgressIndicator(SimpleProgressIndicator):
     # hard to decipher what test is running when only the filename is printed.
     prefix = abspath(join(dirname(__file__), '../test')) + os.sep
     command = output.command[-1]
-    if command.endswith('.js'): command = command[:-3]
-    if command.startswith(prefix): command = command[len(prefix):]
-    command = command.replace('\\', '/')
+    command = NormalizePath(command, prefix)
 
     stdout = output.output.stdout.strip()
     printed_file = False
@@ -1509,12 +1505,16 @@ def SplitPath(s):
   stripped = [ c.strip() for c in s.split('/') ]
   return [ Pattern(s) for s in stripped if len(s) > 0 ]
 
-def NormalizePath(path):
+def NormalizePath(path, prefix='test/'):
   # strip the extra path information of the specified test
-  if path.startswith('test/'):
-    path = path[5:]
+  prefix = prefix.replace('\\', '/')
+  path = path.replace('\\', '/')
+  if path.startswith(prefix):
+    path = path[len(prefix):]
   if path.endswith('.js'):
     path = path[:-3]
+  elif path.endswith('.mjs'):
+    path = path[:-4]
   return path
 
 def GetSpecialCommandProcessor(value):