Don't loose comments when converting ESM to commonjs

packages/babel-helper-module-transforms/src/normalize-and-load-metadata.ts

@@ -54,6 +54,8 @@ export interface SourceModuleMetadata {
     loc: t.SourceLocation | undefined | null;
   };
   lazy?: Lazy;
+  leadingComments?: t.Comment[];
+  trailingComments?: t.Comment[];
 }
 
 export interface LocalExportMetadata {
@@ -245,7 +247,17 @@ function getModuleMetadata(
   );
 
   const sourceData = new Map();
-  const getData = (sourceNode: t.StringLiteral) => {
+  const getData = ({
+    source: sourceNode,
+    loc,
+    leadingComments,
+    trailingComments,
+  }: {
+    source: t.StringLiteral;
+    loc?: t.SourceLocation;
+    leadingComments?: t.Comment[];
+    trailingComments?: t.Comment[];
+  }) => {
     const source = sourceNode.value;
 
     let data = sourceData.get(source);
@@ -271,16 +283,24 @@ function getModuleMetadata(
         lazy: false,
 
         source,
+
+        leadingComments: null,
       };
       sourceData.set(source, data);
     }
+    if (!data.loc) data.loc = loc;
+    if (leadingComments) {
+      (data.leadingComments ||= []).push(...leadingComments);
+    }
+    if (trailingComments) {
+      (data.trailingComments ||= []).push(...trailingComments);
+    }
     return data;
   };
   let hasExports = false;
   programPath.get("body").forEach(child => {
     if (child.isImportDeclaration()) {
-      const data = getData(child.node.source);
-      if (!data.loc) data.loc = child.node.loc;
+      const data = getData(child.node);
 
       child.get("specifiers").forEach(spec => {
         if (spec.isImportDefaultSpecifier()) {
@@ -329,16 +349,16 @@ function getModuleMetadata(
       });
     } else if (child.isExportAllDeclaration()) {
       hasExports = true;
-      const data = getData(child.node.source);
-      if (!data.loc) data.loc = child.node.loc;
+      const data = getData(child.node);
 
       data.reexportAll = {
         loc: child.node.loc,
       };
     } else if (child.isExportNamedDeclaration() && child.node.source) {
       hasExports = true;
-      const data = getData(child.node.source);
-      if (!data.loc) data.loc = child.node.loc;
+      const data = getData(
+        child.node as t.ExportNamedDeclaration & { source: t.StringLiteral },
+      );
 
       child.get("specifiers").forEach(spec => {
         assertExportSpecifier(spec);

packages/babel-plugin-transform-modules-commonjs/src/index.ts

@@ -258,6 +258,8 @@ export default declare((api, options: Options) => {
               }
             }
             header.loc = metadata.loc;
+            header.leadingComments = metadata.leadingComments;
+            header.trailingComments = metadata.trailingComments;
 
             headers.push(header);
             headers.push(

packages/babel-plugin-transform-modules-commonjs/test/fixtures/interop/export-all/output.js

@@ -5,6 +5,9 @@ Object.defineProperty(exports, "__esModule", {
 });
 var _exportNames = {};
 exports.default = void 0;
+// The fact that this exports both a normal default, and all of the names via
+// re-export is an edge case that is important not to miss. See
+// https://github.com/babel/babel/issues/8306 as an example.
 var _react = babelHelpers.interopRequireWildcard(require("react"));
 Object.keys(_react).forEach(function (key) {
   if (key === "default" || key === "__esModule") return;
@@ -17,8 +20,5 @@ Object.keys(_react).forEach(function (key) {
     }
   });
 });
-// The fact that this exports both a normal default, and all of the names via
-// re-export is an edge case that is important not to miss. See
-// https://github.com/babel/babel/issues/8306 as an example.
 var _default2 = _react.default;
 exports.default = _default2;

packages/babel-plugin-transform-modules-commonjs/test/fixtures/misc/comments-2/input.mjs

@@ -0,0 +1,4 @@
+/* istanbul ignore file */
+import * as Something from 'some-dep';
+// a comment
+// export default function() {}

packages/babel-plugin-transform-modules-commonjs/test/fixtures/misc/comments-2/options.json

@@ -0,0 +1,4 @@
+{
+  "externalHelpers": true,
+  "plugins": ["transform-modules-commonjs"]
+}

packages/babel-plugin-transform-modules-commonjs/test/fixtures/misc/comments-2/output.js

@@ -0,0 +1,6 @@
+"use strict";
+
+/* istanbul ignore file */
+var Something = babelHelpers.interopRequireWildcard(require("some-dep"));
+// a comment
+// export default function() {}

packages/babel-plugin-transform-modules-commonjs/test/fixtures/misc/comments/input.mjs

@@ -0,0 +1,11 @@
+// a comment
+import { useState } from "react";
+
+// another comment
+import "react-dom";
+
+/* a third comment */
+import * as R from "react";
+
+// a class
+class A extends useState(R) {}

packages/babel-plugin-transform-modules-commonjs/test/fixtures/misc/comments/options.json

@@ -0,0 +1,4 @@
+{
+  "externalHelpers": true,
+  "plugins": ["transform-modules-commonjs"]
+}

packages/babel-plugin-transform-modules-commonjs/test/fixtures/misc/comments/output.js

@@ -0,0 +1,12 @@
+"use strict";
+
+// a comment
+
+/* a third comment */
+var R = babelHelpers.interopRequireWildcard(require("react"));
+
+// another comment
+
+// a class
+require("react-dom");
+class A extends (0, R.useState)(R) {}