esm: support loader hook transformSource

Refs: nodejs/modules#390

Author: coreyfarrell

doc/api/esm.md

@@ -776,6 +776,17 @@ 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.
 
+### Transform hook
+
+This hook is called only for modules that return `format: 'module'` from
+the `resolve` hook.
+
+```js
+export async function transformSource(url, source) {
+  return source.replace(/'original'/, "'replacement'");
+}
+```
+
 ## Resolution Algorithm
 
 ### Features

lib/internal/modules/esm/loader.js

@@ -61,6 +61,10 @@ class Loader {
     // an object with the same keys as `exports`, whose values are get/set
     // functions for the actual exported values.
     this._dynamicInstantiate = undefined;
+    // These hooks are called FILO when resolve(...).format is 'module' and
+    // has the signature
+    //   (code : string, url : string) -> Promise<code : string>
+    this._transformSource = [];
     // The index for assigning unique URLs to anonymous module evaluation
     this.evalIndex = 0;
   }
@@ -133,14 +137,26 @@ class Loader {
     return module.getNamespace();
   }
 
-  hook({ resolve, dynamicInstantiate }) {
+  async transformSource(url, code) {
+    for (const transformFn of this._transformSource) {
+      code = await transformFn(url, code);
+    }
+
+    return code;
+  }
+
+  hook({ resolve, dynamicInstantiate, transformSource }) {
     // Use .bind() to avoid giving access to the Loader instance when called.
     if (resolve !== undefined)
       this._resolve = FunctionPrototype.bind(resolve, null);
     if (dynamicInstantiate !== undefined) {
       this._dynamicInstantiate =
         FunctionPrototype.bind(dynamicInstantiate, null);
     }
+    if (transformSource !== undefined) {
+      // this.transformSource protects `this`, no need to bind.
+      this._transformSource.push(transformSource);
+    }
   }
 
   async getModuleJob(specifier, parentURL) {

lib/internal/modules/esm/translators.js

@@ -76,7 +76,7 @@ async function importModuleDynamically(specifier, { url }) {
 
 // Strategy for loading a standard JavaScript module
 translators.set('module', async function moduleStrategy(url) {
-  const source = `${await getSource(url)}`;
+  const source = await this.transformSource(url, `${await getSource(url)}`);
   maybeCacheSourceMap(url, source);
   debug(`Translating StandardModule ${url}`);
   const module = new ModuleWrap(source, url);

test/es-module/test-esm-transform-source.mjs

@@ -0,0 +1,10 @@
+// Flags: --experimental-modules --experimental-loader ./test/fixtures/es-module-loaders/transform-loader.mjs
+/* eslint-disable node-core/require-common-first, node-core/required-modules */
+import {
+  foo,
+  bar
+} from '../fixtures/es-module-loaders/module-named-exports.mjs';
+import assert from 'assert';
+
+assert.strictEqual(foo, 'transformed-foo');
+assert.strictEqual(bar, 'transformed-bar');

test/fixtures/es-module-loaders/transform-loader.mjs

@@ -0,0 +1,11 @@
+import { promisify } from 'util';
+
+const delay = promisify(setTimeout);
+
+export async function transformSource(url, source) {
+  await delay(50);
+
+  return source
+    .replace(/'bar'/, "'transformed-bar'")
+    .replace(/'foo'/, "'transformed-foo'");
+}