Use import/extensions eslint plugin

.eslintrc.cjs

@@ -43,6 +43,7 @@ module.exports = {
         "@babel/development/no-undefined-identifier": "error",
         "@babel/development/no-deprecated-clone": "error",
         "guard-for-in": "error",
+        "import/extensions": ["error", { json: "always", cjs: "always" }],
       },
       globals: { PACKAGE_JSON: "readonly" },
     },
@@ -64,6 +65,7 @@ module.exports = {
         "jest/no-standalone-expect": "off",
         "jest/no-test-callback": "off",
         "jest/valid-describe": "off",
+        "import/extensions": ["error", { json: "always", cjs: "always" }],
       },
     },
     {

packages/babel-helper-member-expression-to-functions/src/index.js

@@ -1,5 +1,5 @@
 import * as t from "@babel/types";
-import { willPathCastToBoolean } from "./util.js";
+import { willPathCastToBoolean } from "./util";
 
 class AssignmentMemoiser {
   constructor() {

packages/babel-helper-validator-option/src/validator.ts

@@ -1,4 +1,4 @@
-import { findSuggestion } from "./find-suggestion.js";
+import { findSuggestion } from "./find-suggestion";
 
 export class OptionValidator {
   declare descriptor: string;

packages/babel-parser/src/parser/error.js

@@ -18,7 +18,7 @@ type ErrorContext = {
 
 export type ParsingError = SyntaxError & ErrorContext;
 
-export { ErrorMessages as Errors } from "./error-message.js";
+export { ErrorMessages as Errors } from "./error-message";
 
 export default class ParserError extends CommentsParser {
   // Forward-declaration: defined in tokenizer/index.js

packages/babel-parser/src/parser/expression.js

@@ -53,7 +53,7 @@ import {
   newArrowHeadScope,
   newAsyncArrowScope,
   newExpressionScope,
-} from "../util/expression-scope.js";
+} from "../util/expression-scope";
 import { Errors } from "./error";
 
 export default class ExpressionParser extends LValParser {

packages/babel-plugin-proposal-class-static-block/test/plugin-ordering.test.js

@@ -1,5 +1,5 @@
 import * as babel from "@babel/core";
-import proposalClassStaticBlock from "../lib/index.js";
+import proposalClassStaticBlock from "..";
 
 describe("plugin ordering", () => {
   it("should throw when @babel/plugin-proposal-class-static-block is after class features plugin", () => {

packages/babel-plugin-proposal-optional-chaining/src/index.js

@@ -5,10 +5,7 @@ import {
 } from "@babel/helper-skip-transparent-expression-wrappers";
 import syntaxOptionalChaining from "@babel/plugin-syntax-optional-chaining";
 import { types as t, template } from "@babel/core";
-import {
-  willPathCastToBoolean,
-  findOutermostTransparentParent,
-} from "./util.js";
+import { willPathCastToBoolean, findOutermostTransparentParent } from "./util";
 
 const { ast } = template.expression;
 

packages/babel-plugin-transform-react-jsx-development/src/index.js

@@ -1,3 +1,3 @@
 /* eslint-disable @babel/development/plugin-name */
 
-export { default } from "@babel/plugin-transform-react-jsx/lib/development.js";
+export { default } from "@babel/plugin-transform-react-jsx/lib/development";

packages/babel-plugin-transform-react-jsx/src/development.js

@@ -1,4 +1,4 @@
-import createPlugin from "./create-plugin.js";
+import createPlugin from "./create-plugin";
 
 export default createPlugin({
   name: "transform-react-jsx/development",

packages/babel-plugin-transform-react-jsx/src/index.js

@@ -1,6 +1,6 @@
 /* eslint-disable @babel/development/plugin-name */
 
-import createPlugin from "./create-plugin.js";
+import createPlugin from "./create-plugin";
 
 export default createPlugin({
   name: "transform-react-jsx",

packages/babel-preset-env/src/normalize-options.js

@@ -1,5 +1,5 @@
 // @flow
-import corejs3Polyfills from "core-js-compat/data";
+import corejs3Polyfills from "core-js-compat/data.json";
 import { coerce, SemVer } from "semver";
 import corejs2Polyfills from "@babel/compat-data/corejs2-built-ins";
 import { plugins as pluginsList } from "./plugins-compat-data";

packages/babel-preset-env/src/polyfills/corejs3/entry-plugin.js

@@ -1,7 +1,7 @@
 // @flow
 
-import corejs3Polyfills from "core-js-compat/data";
-import corejsEntries from "core-js-compat/entries";
+import corejs3Polyfills from "core-js-compat/data.json";
+import corejsEntries from "core-js-compat/entries.json";
 import getModulesListForTargetVersion from "core-js-compat/get-modules-list-for-target-version";
 import { filterItems } from "@babel/helper-compilation-targets";
 import {

packages/babel-preset-env/src/polyfills/corejs3/usage-plugin.js

@@ -1,6 +1,6 @@
 // @flow
 
-import corejs3Polyfills from "core-js-compat/data";
+import corejs3Polyfills from "core-js-compat/data.json";
 import corejs3ShippedProposalsList from "@babel/compat-data/corejs3-shipped-proposals";
 import getModulesListForTargetVersion from "core-js-compat/get-modules-list-for-target-version";
 import { filterItems } from "@babel/helper-compilation-targets";

packages/babel-preset-typescript/src/index.js

@@ -1,6 +1,6 @@
 import { declare } from "@babel/helper-plugin-utils";
 import transformTypeScript from "@babel/plugin-transform-typescript";
-import normalizeOptions from "./normalize-options.js";
+import normalizeOptions from "./normalize-options";
 
 export default declare((api, opts) => {
   api.assertVersion(7);

scripts/rollup-plugin-babel-source.js

@@ -88,13 +88,14 @@ export default function () {
         ? packageJson["browser"]
         : packageJson["main"];
 
-      const asJS = path.normalize(
+      let asJS = path.normalize(
         path.join(
           packageFolder,
           // replace lib with src in the package.json entry
           filename.replace(/^(\.\/)?lib\//, "src/")
         )
       );
+      if (!/\.[a-z]+$/.test(asJS)) asJS += ".js";
       const asTS = asJS.replace(/\.js$/, ".ts");
 
       try {