Improved Babel config.

Includes a workaround for babel/babel#8462 .

.size-limit.json

@@ -9,6 +9,6 @@
     "name": "Browser",
     "path": "size-limit-entries/browser.mjs",
     "limit": "2.5 KB",
-    "ignore": ["prop-types", "object-assign"]
+    "ignore": ["prop-types"]
   }
 ]

babelPluginAddBabelRuntimeFileExtensions.js

@@ -0,0 +1,26 @@
+'use strict'
+
+/**
+ * A Babel plugin that adds missing `.js` file extensions to Babel runtime
+ * import specifiers.
+ * @param {object} babel Current Babel object.
+ * @returns {object} Babel plugin object.
+ * @see [babel/babel#8462](https://github.com/babel/babel/issues/8462).
+ * @ignore
+ */
+module.exports = function babelPluginAddBabelRuntimeFileExtensions({ types }) {
+  return {
+    visitor: {
+      'ImportDeclaration|ExportNamedDeclaration'(path) {
+        if (
+          path.node.source &&
+          path.node.source.value.startsWith('@babel/runtime/helpers/') &&
+          !path.node.source.value.endsWith('.js')
+        )
+          path
+            .get('source')
+            .replaceWith(types.stringLiteral(`${path.node.source.value}.js`))
+      }
+    }
+  }
+}

changelog.md

@@ -10,6 +10,7 @@
 ### Patch
 
 - Updated tests for compatibility with updated dependencies.
+- Removed the [`object-assign`](https://npm.im/object-assign) dependency and several Babel dev dependencies after simplifying the Babel config.
 - Added a new [`babel-plugin-transform-require-extensions`](https://npm.im/babel-plugin-transform-require-extensions) dev dependency and ensured ESM import specifiers in both source and published `.mjs` files contain file names with extensions, which [are mandatory in the final Node.js ESM implementation](https://nodejs.org/api/esm.html#esm_mandatory_file_extensions). Published CJS `.js` files now also have file extensions in `require` paths.
 - Stop using [`husky`](https://npm.im/husky) and [`lint-staged`](https://npm.im/lint-staged).
 - Tidied Babel configs.

package.json

@@ -45,20 +45,17 @@
     "extract-files": "^7.0.0",
     "fnv1a": "^1.0.1",
     "mitt": "^1.1.3",
-    "object-assign": "^4.1.1",
     "prop-types": "^15.7.2"
   },
   "devDependencies": {
     "@babel/cli": "^7.8.4",
     "@babel/core": "^7.9.0",
     "@babel/plugin-proposal-class-properties": "^7.8.3",
-    "@babel/plugin-proposal-object-rest-spread": "^7.9.0",
     "@babel/plugin-transform-runtime": "^7.9.0",
     "@babel/preset-env": "^7.9.0",
     "@babel/preset-react": "^7.9.1",
     "@size-limit/preset-small-lib": "^4.4.0",
     "babel-eslint": "^10.1.0",
-    "babel-plugin-transform-replace-object-assign": "^2.0.0",
     "babel-plugin-transform-require-extensions": "^2.0.0",
     "cross-fetch": "^3.0.4",
     "eslint": "^6.8.0",

src/server/.babelrc.js

@@ -1,8 +1,13 @@
 'use strict'
 
+const plugins = ['@babel/transform-runtime', 'transform-require-extensions']
+
+if (process.env.BABEL_ESM)
+  plugins.push(require('../../babelPluginAddBabelRuntimeFileExtensions.js'))
+
 module.exports = {
   comments: false,
-  plugins: ['@babel/transform-runtime', 'transform-require-extensions'],
+  plugins,
   presets: [
     [
       '@babel/env',

src/test/.babelrc.js

@@ -1,8 +1,13 @@
 'use strict'
 
+const plugins = ['@babel/transform-runtime', 'transform-require-extensions']
+
+if (process.env.BABEL_ESM)
+  plugins.push(require('../../babelPluginAddBabelRuntimeFileExtensions.js'))
+
 module.exports = {
   comments: false,
-  plugins: ['@babel/transform-runtime', 'transform-require-extensions'],
+  plugins,
   presets: [
     [
       '@babel/env',

src/universal/.babelrc.js

@@ -1,15 +1,18 @@
 'use strict'
 
+const plugins = [
+  ['@babel/proposal-class-properties', { loose: true }],
+  '@babel/transform-runtime',
+  'transform-require-extensions'
+]
+
+if (process.env.BABEL_ESM)
+  plugins.push(require('../../babelPluginAddBabelRuntimeFileExtensions.js'))
+
 module.exports = {
   comments: false,
-  plugins: [
-    ['@babel/proposal-object-rest-spread', { loose: true, useBuiltIns: true }],
-    ['@babel/proposal-class-properties', { loose: true }],
-    '@babel/transform-runtime',
-    'transform-require-extensions'
-  ],
+  plugins,
   presets: [
-    { plugins: ['transform-replace-object-assign'] },
     [
       '@babel/env',
       {
@@ -19,6 +22,6 @@ module.exports = {
         loose: true
       }
     ],
-    ['@babel/react', { useBuiltIns: true }]
+    '@babel/react'
   ]
 }